Cut /api/tasks/ p99 from ~2500ms toward ~150-300ms

Stack of optimizations against the same Hetzner→Neon transatlantic link.
The trace revealed every visible ms was network/proxy overhead — DB
execution itself is sub-millisecond per query (verified via EXPLAIN
ANALYZE: index scans on every hot path).

Connection layer:
- DB_HOST → Neon pooler endpoint (-pooler suffix). PgBouncer
  transaction-mode keeps backend Postgres connections warm so we no
  longer pay the ~110ms Postgres-startup RTT on cold queries.
- GORM pool tuned: MaxIdleConns 10→20, MaxLifetime 600s→1800s,
  MaxIdleTime added (default 0 = never close idle).
- Eager pool warm-up at boot via parallel pings — first user request
  no longer pays the ~440ms TCP+TLS+startup handshake.
- Redis maxmemory-policy noeviction → allkeys-lru. Cache writes will
  evict cold keys instead of erroring at the 256MB limit.

Auth layer:
- TokenCacheTTL 5min → 1 hour (Redis token cache).
- UserCacheTTL 30s → 5min (in-memory User cache, per pod).
- UserCache gains a 5,000-entry LRU cap so a flood of unique users
  can't blow up pod RSS. ~5MB worst-case per pod.
- Token + user lookup collapsed from 2 GORM Preload queries into a
  single INNER JOIN. Saves 1 RTT per cold-cache request.
- Auth middleware's m.db.* now use db.WithContext(ctx) so the SQL
  spans nest under the parent HTTP request in Jaeger.

Service layer:
- TaskService.ListTasks: replaced two-step
  FindResidenceIDsByUser → GetKanbanDataForMultipleResidences
  with a single GetKanbanDataForUser that uses a Postgres subquery
  for residence-access. One round-trip instead of two.
- New CacheService residence-IDs cache: \"residence_ids_user:<id>\"
  with 5-min TTL. Wired into Task/Residence/Contractor/Document
  services for the four hot read paths that need this list.
- Cache invalidation on every relevant mutation: CreateResidence,
  DeleteResidence, JoinWithCode, RemoveUser. DeleteResidence
  invalidates every member of the residence, not just the owner.

What this stacks up to (Hetzner→Neon, before US migration):
  Path                                 Before        After (target)
  Cache-warm authed read               ~800ms        ~100-200ms
  Cache-cold authed read (1st in 1hr)  ~2500ms       ~500-700ms
  First request after deploy           ~2500ms       ~700-900ms

The endgame US-region migration on top of this gets us to ~30-50ms
warm-cache, but we're shippable at ~150ms warm right now.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/services/residence_id_cache.go

@@ -0,0 +1,41 @@
+package services
+
+import (
+	"context"
+
+	"github.com/treytartt/honeydue-api/internal/repositories"
+)
+
+// cachedResidenceIDsForUser fetches the residence-ID list for a user, going
+// through Redis (5-min TTL) before falling back to Postgres.
+//
+// Used on every authed read path (tasks, documents, contractors, summary)
+// because the list rarely changes — only on share-code accept, member
+// removal, or residence delete. Callers must invalidate after mutations
+// via cache.InvalidateResidenceIDsForUsers.
+//
+// A nil cache is permitted — the function falls through to the repo
+// directly, so this works in tests and in failure modes.
+func cachedResidenceIDsForUser(
+	ctx context.Context,
+	cache *CacheService,
+	residenceRepo *repositories.ResidenceRepository,
+	userID uint,
+) ([]uint, error) {
+	if cache != nil {
+		if ids, err := cache.GetCachedResidenceIDsForUser(ctx, userID); err == nil {
+			return ids, nil
+		}
+	}
+
+	ids, err := residenceRepo.WithContext(ctx).FindResidenceIDsByUser(userID)
+	if err != nil {
+		return nil, err
+	}
+
+	if cache != nil {
+		// Best-effort cache fill; don't fail the request on Redis hiccup.
+		_ = cache.CacheResidenceIDsForUser(ctx, userID, ids)
+	}
+	return ids, nil
+}