feat: security hardening, DB indexes, token revocation, and input validation

- SSRF protection: DNS resolution + private IP blocking on metadata fetcher
- Upload security: CSP/X-Frame-Options headers, SVG forced download, nosniff
- Auth hardening: JWT secret min length, password min 8 chars, token revocation via password_changed_at
- Attachment ownership verification before linking to messages
- Message length limit (4000 chars) enforced on client and server
- Asset URL validation on avatar/banner updates
- Federation instance validation (domain regex, origin scheme, length limits)
- DB indexes on all FK columns for query performance
- Migrations: nullable moderator columns, dm_messages reply_to FK constraint
- File cleanup on avatar/banner replacement and space deletion
- Fastify trustProxy, AbortController on fetches, typing map size cap

packages/shared/src/types.ts

@@ -1,3 +1,7 @@
+// ─── Constants ──────────────────────────────────────────────────────────────
+
+export const MAX_MESSAGE_LENGTH = 4000;
+
 // ─── User Types ─────────────────────────────────────────────────────────────
 
 export const AVATAR_COLORS = ['mint', 'sky', 'lavender', 'coral', 'rose', 'teal', 'amber'] as const;