fix: close eight regressions the audit pass found, five of them mine

Two adversarial reviews over 61119be, 1d9fb11 and eb0e405. The merge itself came
back clean — client_upload_id end to end, TempFileGuard's arm/retarget/disarm, the
supervised sweep wiring and v_feed's column parity were all verified sound. What
follows is what my own three commits broke.

BLOCKER — a post-release rebuild was permanently impossible, and it 404'd the keepsake

1d9fb11 deferred prune_superseded_archives to run only on success, so a failed
rebuild could no longer destroy the last good archive. It did not follow that
through: ensure_export_space runs BEFORE the prune, so at rebuild time the previous
generation is still on disk and counted against free. That halves the gallery a
rebuild can survive (~4.6 GB) relative to what the upload gate accepts (~7.8 GB) —
and it self-locks, because invalidate_and_arm bumps the epoch on COMMIT, which 404s
both download routes immediately, while the only code that could free the space now
runs only after a success that can never happen. A guest deleting their own photo is
enough to trigger it. Recovery needed `docker exec rm`.

Now two-phase: try to build while preserving the old generation; if that genuinely
does not fit, reclaim it and try once more. Strictly better than both the original
ordering and my change — the old archive is sacrificed only when it is the only way
to get a new one.

BLOCKER — the deferred prune could delete the last archive when a worker LOST the race

run_*_export_inner returned Ok(()) on the superseded/discard path, so `res.is_ok()`
fired the prune with the worker's own RETIRED epoch as keep_seq. At that moment the
winning generation is still `pending` with no file, so protected_files is empty and
the last good archive was deleted with no replacement. Exactly the invariant
deferring the prune was meant to establish. Returns Err(Superseded) now, which
abandon_if_superseded already swallows for the caller.

BLOCKER — the low-disk banner could never fire before the wall

eb0e405's gate refuses at `free < keepsake + DISK_RESERVE`, while disk_is_low warned
at `free < keepsake`. The two differ by the whole reserve, so the wall always came
first: every guest blocked from uploading while the host dashboard showed ~27 GB free
and no banner, with nobody on site. disk_is_low now shares the gate's expression plus
a 25% margin, and a test asserts the banner fires at the gate threshold across the
whole gallery-size range.

BLOCKER — I raised the unauthenticated bcrypt ceiling 24x on a 2 vCPU box

1d9fb11 moved admin_login's tight bucket after verify_password (correct — that is what
stops a guest locking the operator out) but replaced the incidental 5/min bound on
bcrypt with 120/min and nothing global. bcrypt is on spawn_blocking, but tokio's
blocking pool is 512 threads, so "off the runtime" is not "bounded": enough concurrent
verifies preempt both async workers and uploads, feed and SSE stall. Three
unauthenticated endpoints reach bcrypt and every guest shares one NAT IP, so per-IP
limits bound nothing globally. Adds a process-wide semaphore of `cores - 1` around both
verify and hash, and drops the ceiling to 30.

Also correcting my own claim: "a correct password is never throttled" was wrong. The
failure bucket cannot block it, but the CPU ceiling still can. The code comment said so;
the commit message did not.

BLOCKER — migration 025 could crash-loop the app on boot

Its UPDATE derives `Name (8hex)` with no guard against idx_user_event_name_ci. A guest
who had already joined as exactly that string makes the migration fail, which
propagates out of create_pool, exits main, and `restart: unless-stopped` turns it into
a permanent loop — a worse version of the lockout the migration exists to clean up.
Now skips colliding rows (create_admin_user already falls back to Admin-<8hex>, so the
cleanup is convenience, not load-bearing). Also `role = 'guest'` rather than
`<> 'admin'`, which was renaming legitimately promoted hosts named "Host".

DEGRADATION — the watchdog's suspension credit was unbounded

Background tabs are throttled to ~1 tick/min WITHOUT the network stack pausing, and the
tick gap cannot tell that from a freeze. Crediting every late tick grew the observed
silence by only one interval per real minute, so a dead socket took ~18 minutes to
detect while holding the queue's processing latch. Credit is now capped at one stall
window and REFILLS on real progress: an upload that is moving survives any number of
screen locks, while one that is silent and suspended is detected within ~3 minutes.

DEGRADATION — the 4xx log line was an unauthenticated log-injection vector

validate_display_name allowed newlines, several 4xx messages interpolate the name, and
%message wrote it unescaped. Two unauthenticated /join requests could forge arbitrary
lines in the only forensic record an unattended event has. Fixed at both ends: control
characters rejected at the door, and `detail = ?message` escapes on the way out (which
also stops colliding with tracing's reserved `message` field). 401/404 drop to DEBUG —
they carry no operator signal and were the cheapest lines for a scanner to use to roll
the 30 MB log window in minutes.

DEGRADATION — the quota floor was inverted exactly where it mattered

`computed.max(MIN.min(budget))`: `budget` is the whole disk's share, so below 500 MiB
the "floor" became the entire remaining budget and EVERY uploader was authorised all of
it — 400 MB free, 3 uploaders, 300 MB each. A test pinned that as correct under the name
`the_floor_never_exceeds_what_the_disk_can_back`. Both fixed.

Also replaces the headline gate test, which asserted its own precondition inside an `if`
on that precondition and could not fail. It now pins what actually binds the gate to the
preflight — that required_free_bytes charges for both halves — plus the ceiling band.

Verified: 151/151 backend tests against a live Postgres, clippy clean, 58/58 vitest,
svelte-check 0 errors, eslint clean, both builds, caddy validate, and the migration
collision reproduced against Postgres 16 before and after.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/migrations/025_reserved_names_and_pin_decay.up.sql

@@ -13,10 +13,32 @@
 --
 -- RENAMED, NEVER DELETED: the guest keeps their uploads, their PIN and their session. Only
 -- non-admin rows are touched — a real admin row named "Admin" is the expected state.
+-- Two guards that are not optional, because this statement runs INSIDE the migration
+-- transaction on boot and a failure here exits the process — `restart: unless-stopped` then
+-- turns it into a crash loop with no in-app recovery. That is a strictly worse version of the
+-- lockout this migration exists to clean up after.
+--
+--   * role = 'guest', not role <> 'admin'. The enum also has 'host' (001), and hosts are
+--     promoted from guests at runtime — so <> 'admin' renamed a legitimately promoted staff
+--     member whose name happens to be "Host".
+--   * NOT EXISTS. The target name is derived, not unique: `idx_user_event_name_ci` (007) is a
+--     UNIQUE index on (event_id, lower(display_name)), and nothing stopped a second guest from
+--     having already joined as exactly "Admin (a1b2c3d4)" — the old code had no reserved-name
+--     guard and the join response hands each guest their own id. Rare, but the cost of losing
+--     that bet is the whole event.
+--
+-- A row that collides is simply left alone: `create_admin_user` already handles a name clash by
+-- falling back to `Admin-<8hex>`, and `admin_login` no longer resolves by name at all, so this
+-- cleanup is convenience rather than load-bearing.
 UPDATE "user" u
    SET display_name = u.display_name || ' (' || left(u.id::text, 8) || ')'
- WHERE u.role <> 'admin'
-   AND lower(u.display_name) IN ('admin', 'administrator', 'host', 'eventsnap');
+ WHERE u.role = 'guest'
+   AND lower(u.display_name) IN ('admin', 'administrator', 'host', 'eventsnap')
+   AND NOT EXISTS (
+        SELECT 1 FROM "user" x
+         WHERE x.event_id = u.event_id
+           AND lower(x.display_name) = lower(u.display_name || ' (' || left(u.id::text, 8) || ')')
+   );
 
 -- 2. PIN LOCKOUT DECAY.
 --