Implement tasks 60, 61, 62: backups, rate limiting, structured logging

Task 60 — Postgres backups:
- deploy/backups/: backup.sh (pg_dump -Fc nightly), retain.sh (14/8/6 tiers),
  restore.sh, entrypoint.sh, crontab
- famapp-backup Alpine service + backups volume added to deploy/compose.yaml
- Restore procedure in deploy/backups/README.md

Task 61 — Rate limiting on share links:
- src/lib/rate-limit.ts: Edge-compatible sliding-window counter (50/min, LRU eviction)
  with consume(), isRateLimited(), recordFailure() exports
- middleware.ts: enforces 429 with Retry-After: 60 for /s/[token] (IP + token prefix)
- /s/[token]/page.tsx: tracks only failed resolveShareToken calls via recordFailure()

Task 62 — Structured logging:
- pino + pino-pretty installed; serverExternalPackages added to next.config.ts
- src/lib/logger.ts: JSON in production, pretty in dev, level from LOG_LEVEL env
- middleware.ts: structured JSON request log (method, path, status, ms, authenticated)
- _core/push.ts, notify.ts, reminders.ts: console.error/log → logger.error/info

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

deploy/backups/retain.sh

@@ -0,0 +1,24 @@
+#!/bin/sh
+# Prune old backup files according to retention policy.
+# daily: keep 14  weekly: keep 8  monthly: keep 6
+set -eu
+
+prune() {
+    local dir="$1"
+    local keep="$2"
+    [ -d "$dir" ] || return 0
+    # Files are YYYY-MM-DD.dump; lexicographic sort = chronological.
+    # tail skips the N newest; xargs deletes the rest.
+    ls -1 "$dir"/*.dump 2>/dev/null \
+        | sort \
+        | head -n "-$keep" \
+        | xargs -r rm -f --
+}
+
+for db in famapp authentik; do
+    prune "/backups/$db/daily"   14
+    prune "/backups/$db/weekly"   8
+    prune "/backups/$db/monthly"  6
+done
+
+echo "[retain] done"