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/document_service.go

@@ -24,6 +24,7 @@ import (
 type DocumentService struct {
 	documentRepo  *repositories.DocumentRepository
 	residenceRepo *repositories.ResidenceRepository
+	cache         *CacheService
 }
 
 // NewDocumentService creates a new document service
@@ -34,6 +35,11 @@ func NewDocumentService(documentRepo *repositories.DocumentRepository, residence
 	}
 }
 
+// SetCacheService wires Redis caching for residence-ID lookups.
+func (s *DocumentService) SetCacheService(cache *CacheService) {
+	s.cache = cache
+}
+
 // GetDocument gets a document by ID with access check
 func (s *DocumentService) GetDocument(ctx context.Context, documentID, userID uint) (*responses.DocumentResponse, error) {
 	document, err := s.documentRepo.WithContext(ctx).FindByID(documentID)
@@ -60,7 +66,7 @@ func (s *DocumentService) GetDocument(ctx context.Context, documentID, userID ui
 // ListDocuments lists all documents accessible to a user, with optional filters.
 func (s *DocumentService) ListDocuments(ctx context.Context, userID uint, filter *repositories.DocumentFilter) ([]responses.DocumentResponse, error) {
 	// Get residence IDs (lightweight - no preloads)
-	residenceIDs, err := s.residenceRepo.WithContext(ctx).FindResidenceIDsByUser(userID)
+	residenceIDs, err := cachedResidenceIDsForUser(ctx, s.cache, s.residenceRepo, userID)
 	if err != nil {
 		return nil, apperrors.Internal(err)
 	}
@@ -95,7 +101,7 @@ func (s *DocumentService) ListDocuments(ctx context.Context, userID uint, filter
 // ListWarranties lists all warranty documents
 func (s *DocumentService) ListWarranties(ctx context.Context, userID uint) ([]responses.DocumentResponse, error) {
 	// Get residence IDs (lightweight - no preloads)
-	residenceIDs, err := s.residenceRepo.WithContext(ctx).FindResidenceIDsByUser(userID)
+	residenceIDs, err := cachedResidenceIDsForUser(ctx, s.cache, s.residenceRepo, userID)
 	if err != nil {
 		return nil, apperrors.Internal(err)
 	}