Add delete account endpoint and file encryption at rest

Delete Account (Plan #2):
- DELETE /api/auth/account/ with password or "DELETE" confirmation
- Cascade delete across 15+ tables in correct FK order
- Auth provider detection (email/apple/google) for /auth/me/
- File cleanup after account deletion
- Handler + repository tests (12 tests)

Encryption at Rest (Plan #3):
- AES-256-GCM envelope encryption (per-file DEK wrapped by KEK)
- Encrypt on upload, auto-decrypt on serve via StorageService.ReadFile()
- MediaHandler serves decrypted files via c.Blob()
- TaskService email image loading uses ReadFile()
- cmd/migrate-encrypt CLI tool with --dry-run for existing files
- Encryption service + storage service tests (18 tests)

.env.example

@@ -50,6 +50,8 @@ STORAGE_UPLOAD_DIR=./uploads
 STORAGE_BASE_URL=/uploads
 STORAGE_MAX_FILE_SIZE=10485760
 STORAGE_ALLOWED_TYPES=image/jpeg,image/png,image/gif,image/webp,application/pdf
+# 64-char hex key for file encryption at rest. Generate with: openssl rand -hex 32
+STORAGE_ENCRYPTION_KEY=
 
 # Feature Flags (Kill Switches)
 # Set to false to disable. All default to true (enabled).