Fix 113 hardening issues across entire Go backend

Security:
- Replace all binding: tags with validate: + c.Validate() in admin handlers
- Add rate limiting to auth endpoints (login, register, password reset)
- Add security headers (HSTS, XSS protection, nosniff, frame options)
- Wire Google Pub/Sub token verification into webhook handler
- Replace ParseUnverified with proper OIDC/JWKS key verification
- Verify inner Apple JWS signatures in webhook handler
- Add io.LimitReader (1MB) to all webhook body reads
- Add ownership verification to file deletion
- Move hardcoded admin credentials to env vars
- Add uniqueIndex to User.Email
- Hide ConfirmationCode from JSON serialization
- Mask confirmation codes in admin responses
- Use http.DetectContentType for upload validation
- Fix path traversal in storage service
- Replace os.Getenv with Viper in stripe service
- Sanitize Redis URLs before logging
- Separate DEBUG_FIXED_CODES from DEBUG flag
- Reject weak SECRET_KEY in production
- Add host check on /_next/* proxy routes
- Use explicit localhost CORS origins in debug mode
- Replace err.Error() with generic messages in all admin error responses

Critical fixes:
- Rewrite FCM to HTTP v1 API with OAuth 2.0 service account auth
- Fix user_customuser -> auth_user table names in raw SQL
- Fix dashboard verified query to use UserProfile model
- Add escapeLikeWildcards() to prevent SQL wildcard injection

Bug fixes:
- Add bounds checks for days/expiring_soon query params (1-3650)
- Add receipt_data/transaction_id empty-check to RestoreSubscription
- Change Active bool -> *bool in device handler
- Check all unchecked GORM/FindByIDWithProfile errors
- Add validation for notification hour fields (0-23)
- Add max=10000 validation on task description updates

Transactions & data integrity:
- Wrap registration flow in transaction
- Wrap QuickComplete in transaction
- Move image creation inside completion transaction
- Wrap SetSpecialties in transaction
- Wrap GetOrCreateToken in transaction
- Wrap completion+image deletion in transaction

Performance:
- Batch completion summaries (2 queries vs 2N)
- Reuse single http.Client in IAP validation
- Cache dashboard counts (30s TTL)
- Batch COUNT queries in admin user list
- Add Limit(500) to document queries
- Add reminder_stage+due_date filters to reminder queries
- Parse AllowedTypes once at init
- In-memory user cache in auth middleware (30s TTL)
- Timezone change detection cache
- Optimize P95 with per-endpoint sorted buffers
- Replace crypto/md5 with hash/fnv for ETags

Code quality:
- Add sync.Once to all monitoring Stop()/Close() methods
- Replace 8 fmt.Printf with zerolog in auth service
- Log previously discarded errors
- Standardize delete response shapes
- Route hardcoded English through i18n
- Remove FileURL from DocumentResponse (keep MediaURL only)
- Thread user timezone through kanban board responses
- Initialize empty slices to prevent null JSON
- Extract shared field map for task Update/UpdateTx
- Delete unused SoftDeleteModel, min(), formatCron, legacy handlers

Worker & jobs:
- Wire Asynq email infrastructure into worker
- Register HandleReminderLogCleanup with daily 3AM cron
- Use per-user timezone in HandleSmartReminder
- Replace direct DB queries with repository calls
- Delete legacy reminder handlers (~200 lines)
- Delete unused task type constants

Dependencies:
- Replace archived jung-kurt/gofpdf with go-pdf/fpdf
- Replace unmaintained gomail.v2 with wneessen/go-mail
- Add TODO for Echo jwt v3 transitive dep removal

Test infrastructure:
- Fix MakeRequest/SeedLookupData error handling
- Replace os.Exit(0) with t.Skip() in scope/consistency tests
- Add 11 new FCM v1 tests

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/middleware/user_cache.go

@@ -0,0 +1,113 @@
+package middleware
+
+import (
+	"sync"
+	"time"
+
+	"github.com/treytartt/honeydue-api/internal/models"
+)
+
+// userCacheEntry holds a cached user record with an expiration time.
+type userCacheEntry struct {
+	user      *models.User
+	expiresAt time.Time
+}
+
+// UserCache is a concurrency-safe in-memory cache for User records, keyed by
+// user ID. Entries expire after a configurable TTL. The cache uses a sync.Map
+// for lock-free reads on the hot path, with periodic lazy eviction of stale
+// entries during Set operations.
+type UserCache struct {
+	store   sync.Map
+	ttl     time.Duration
+	lastGC  time.Time
+	gcMu    sync.Mutex
+	gcEvery time.Duration
+}
+
+// NewUserCache creates a UserCache with the given TTL for entries.
+func NewUserCache(ttl time.Duration) *UserCache {
+	return &UserCache{
+		ttl:     ttl,
+		lastGC:  time.Now(),
+		gcEvery: 2 * time.Minute,
+	}
+}
+
+// Get returns a cached user by ID, or nil if not found or expired.
+func (c *UserCache) Get(userID uint) *models.User {
+	val, ok := c.store.Load(userID)
+	if !ok {
+		return nil
+	}
+	entry := val.(*userCacheEntry)
+	if time.Now().After(entry.expiresAt) {
+		c.store.Delete(userID)
+		return nil
+	}
+	// Return a shallow copy so callers cannot mutate the cached value.
+	user := *entry.user
+	return &user
+}
+
+// Set stores a user in the cache. It also triggers a background garbage-
+// collection sweep if enough time has elapsed since the last one.
+func (c *UserCache) Set(user *models.User) {
+	// Store a copy to prevent external mutation of the cached object.
+	copied := *user
+	c.store.Store(user.ID, &userCacheEntry{
+		user:      &copied,
+		expiresAt: time.Now().Add(c.ttl),
+	})
+	c.maybeGC()
+}
+
+// Invalidate removes a user from the cache by ID.
+func (c *UserCache) Invalidate(userID uint) {
+	c.store.Delete(userID)
+}
+
+// maybeGC lazily sweeps expired entries at most once per gcEvery interval.
+func (c *UserCache) maybeGC() {
+	c.gcMu.Lock()
+	if time.Since(c.lastGC) < c.gcEvery {
+		c.gcMu.Unlock()
+		return
+	}
+	c.lastGC = time.Now()
+	c.gcMu.Unlock()
+
+	now := time.Now()
+	c.store.Range(func(key, value any) bool {
+		entry := value.(*userCacheEntry)
+		if now.After(entry.expiresAt) {
+			c.store.Delete(key)
+		}
+		return true
+	})
+}
+
+// TimezoneCache tracks the last-known timezone per user ID so the timezone
+// middleware only writes to the database when the value actually changes.
+type TimezoneCache struct {
+	store sync.Map
+}
+
+// NewTimezoneCache creates a new TimezoneCache.
+func NewTimezoneCache() *TimezoneCache {
+	return &TimezoneCache{}
+}
+
+// GetAndCompare returns true if the cached timezone for the user matches tz.
+// If the timezone is different (or not yet cached), it updates the cache and
+// returns false, signaling that a DB write is needed.
+func (tc *TimezoneCache) GetAndCompare(userID uint, tz string) (unchanged bool) {
+	val, loaded := tc.store.Load(userID)
+	if loaded {
+		if cached, ok := val.(string); ok && cached == tz {
+			return true
+		}
+	}
+	tc.store.Store(userID, tz)
+	return false
+}