chore: raise the db memory limit and rate-limit social writes

Two smaller operational items.

POSTGRES 512M -> 1G. DATABASE_MAX_CONNECTIONS is 30 for a ~100-guest event (feed
polling + SSE + uploads at once), and 30 backends plus Postgres 16's default
shared_buffers leaves very little headroom at 512M. An OOM here doesn't degrade one
feature -- every request path touches the database, so it takes the event down.
Memory is the cheaper knob than shrinking the pool back and reintroducing the
queueing it was raised to fix. .env.example now names the pairing explicitly, the way
it already does for COMPRESSION_WORKER_CONCURRENCY.

SOCIAL WRITES WERE UNTHROTTLED. toggle_like, add_comment and delete_comment were the
only mutating endpoints in the app with no limit at all -- upload, join, recover,
export and admin login all carry one. Asymmetric coverage rather than a deliberate
decision.

Low severity, and honestly so: a like fans an SSE broadcast to every client, but the
export regeneration a comment deletion triggers is contained (REGEN_DEBOUNCE 20s,
workers born with their epoch, superseded ones inert). So the ceiling is 120/min --
far above anything a real guest produces. This bounds a script, not an enthusiastic
double-tapper.

ONE bucket across all three actions: separate buckets would let a caller triple the
aggregate write rate by alternating between them. Keyed per USER, matching the feed
and upload limits -- at a venue every guest is behind one NAT, and an IP key is what
made the /join and /feed limits turn guests away in the first place.

Migration 020 seeds both keys, and both are wired into the admin allowlist, the
config UI and the e2e reseed -- the step two earlier per-area toggles missed, which
left switches that existed in code and could never be flipped.

Tests: 4 e2e, including that the shared bucket really is shared (the part most likely
to be lost in a refactor) and that one guest hitting the ceiling doesn't block
another behind the same IP.

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

backend/src/handlers/test_admin.rs

@@ -63,6 +63,7 @@ pub async fn truncate_all(
             ('export_rate_per_day',        '3'),
             ('join_ip_rate_per_min',       '60'),
             ('recover_ip_rate_per_min',    '30'),
+            ('social_rate_per_min',        '120'),
             ('quota_tolerance',            '0.75'),
             ('estimated_guest_count',      '100'),
             ('compression_concurrency',    '2'),
@@ -71,6 +72,7 @@ pub async fn truncate_all(
             ('feed_rate_enabled',          'false'),
             ('export_rate_enabled',        'false'),
             ('join_rate_enabled',          'false'),
+            ('social_rate_enabled',        'false'),
             ('admin_login_rate_enabled',   'false'),
             ('quota_enabled',              'false'),
             ('storage_quota_enabled',      'false'),