docs: add a restore procedure, fix the backup cadence, and correct quota_tolerance

Four things, all found by the same question: what does an operator standing at the
venue actually need?

A RESTORE PROCEDURE. There was none anywhere, and a backup you have never restored
isn't a backup. Two hazards worth writing down: media must be extracted preserving
ownership (the app runs as uid 100 / gid 101, and a root-owned restore makes every
upload fail with EACCES surfacing as a generic 500), and the app must be STOPPED
first, because migrations run on boot and a live pool will fight the restore.

Both the backup and the restore commands were run against the real stack before being
written down, which caught two that would have failed:

  - The plain `pg_dump` did not restore: `psql` aborted on `ERROR: schema
    "_sqlx_test" already exists`. pg_dump emits no DROPs without --clean --if-exists,
    so the documented dump could only ever be restored into an empty database. Fixed
    at the source (the dump is now self-cleaning) and verified end to end: 16 tables
    back, exit 0.
  - `--same-owner` does not exist in BusyBox tar, which is what `alpine` ships, so
    the extract aborted before unpacking anything. `--numeric-owner` plus the
    explicit chown, verified to land 100:101.

BACKUP CADENCE. "Weekly offsite" is the wrong shape when every irreplaceable byte is
created in one eight-hour window and nobody can retake a wedding. The backup that
matters runs that night, and again after the release so the keepsake is captured.
Also: take the DB dump and the media tarball back to back, or you get rows pointing
at files the dump doesn't know about.

quota_tolerance WAS DOCUMENTED AS SOMETHING IT ISN'T. .env.example called it "fraction
of disk that triggers the low-storage warning". It is the multiplier in
`floor(free_disk * tolerance / active_uploaders)` -- so an operator who wants "warn me
later" and sets 0.95 is actually authorising guests to fill 95% of the disk, moving
the fixed point from 43% to ~49% and eating the export headroom. The admin UI labelled
it "Toleranz (0-1)" with no explanation at all, which invites exactly that reading;
it is now "Speicher-Anteil für Gäste" with the formula in the hint. Wrong docs on a
tuning knob are worse than no docs.

SIZING. New section with the arithmetic: three volumes on one filesystem, the quota
fixed point at tolerance/(1+tolerance), and the fact the 80 GB baseline does not cover
the keepsake -- both archives are built concurrently and each is roughly a second copy
of every original. Provision ~3x expected media, or give exports its own volume.

Also ticks the low-disk alert off the roadmap, since it now exists.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.env.example

@@ -54,8 +54,26 @@ EXPORT_PATH=/exports
 #   max image size    20 MB
 #   max video size   500 MB
 #   estimated guests 100
-#   quota tolerance  0.75  (fraction of disk that triggers the low-storage warning)
+#   quota tolerance  0.75  (see below — NOT a warning threshold)
 # Adjust these in the admin UI before the event if needed.
+#
+# quota_tolerance is the MULTIPLIER IN THE PER-USER QUOTA FORMULA, not the point at
+# which anything warns you:
+#
+#     per_user_limit = floor(free_disk * quota_tolerance / active_uploaders)
+#
+# It is recomputed against LIVE free space on every upload, so it self-throttles: guests
+# converge on a fixed point at tolerance/(1+tolerance) of the free space you started
+# with — 43% at 0.75, i.e. ~30 GB of a fresh 70 GB.
+#
+# Raising it therefore AUTHORISES GUESTS TO FILL MORE OF THE DISK. Setting 0.95 in the
+# belief that it means "warn me later" moves the fixed point to ~49% and eats the
+# headroom the keepsake needs — and the keepsake needs a lot, because Gallery.zip and
+# Memories.zip are each roughly a second copy of every original (both store media
+# uncompressed). Budget for media + 2x media, or move exports to their own volume.
+#
+# 0.75 is the tested default. Lower it if the box is tight; raise it only if you have
+# provisioned export headroom separately.
 
 # ── Workers ───────────────────────────────────────────────────────────────────
 # Number of parallel image/video compression workers. Default 2. This is the main