Initial CloudLink scaffold: E2E encrypted PWA with messaging and cloud drive

- Zero-knowledge architecture: all encryption/decryption client-side via Web Crypto API
- AES-GCM 256 for messages, files, and key wrapping; PBKDF2 for password-based key derivation
- Node.js/TypeScript backend: JWT auth with refresh rotation, argon2id passwords, TOTP 2FA
- PostgreSQL schema: encrypted blobs only, server never stores plaintext
- Real-time messaging via WebSocket relay (server routes ciphertext, cannot read it)
- Encrypted file upload/download with per-file keys wrapped by user master key
- React PWA frontend: Vite + Tailwind + service worker for offline support
- Docker Compose: postgres + backend + frontend + nginx with TLS

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/db/schema.sql

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+CREATE EXTENSION IF NOT EXISTS "pgcrypto";
+
+-- Users: minimal plaintext metadata, everything sensitive is client-encrypted
+CREATE TABLE IF NOT EXISTS users (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  username TEXT UNIQUE NOT NULL,
+  email TEXT UNIQUE NOT NULL,
+  password_hash TEXT NOT NULL,
+  -- Argon2id salt used client-side for key derivation (base64)
+  kdf_salt TEXT NOT NULL,
+  -- Client-side encrypted master key (base64 ciphertext + nonce)
+  encrypted_master_key TEXT NOT NULL,
+  -- TOTP secret (server-side, not part of user's encrypted data)
+  totp_secret TEXT,
+  totp_enabled BOOLEAN NOT NULL DEFAULT false,
+  -- Backup codes for 2FA (hashed)
+  totp_backup_codes TEXT[] DEFAULT '{}',
+  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  last_seen_at TIMESTAMPTZ
+);
+
+-- Refresh tokens for JWT rotation
+CREATE TABLE IF NOT EXISTS refresh_tokens (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  token_hash TEXT NOT NULL UNIQUE,
+  expires_at TIMESTAMPTZ NOT NULL,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+-- Direct message conversations
+CREATE TABLE IF NOT EXISTS conversations (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE TABLE IF NOT EXISTS conversation_members (
+  conversation_id UUID NOT NULL REFERENCES conversations(id) ON DELETE CASCADE,
+  user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  -- Per-conversation encrypted key (client encrypts shared key with each member's public key)
+  encrypted_key TEXT NOT NULL,
+  joined_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  PRIMARY KEY (conversation_id, user_id)
+);
+
+-- Messages: ciphertext only — server cannot read content
+CREATE TABLE IF NOT EXISTS messages (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  conversation_id UUID NOT NULL REFERENCES conversations(id) ON DELETE CASCADE,
+  sender_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  -- AES-GCM encrypted content (base64: nonce + ciphertext + tag)
+  ciphertext TEXT NOT NULL,
+  -- Optional: encrypted filename if message contains a file reference
+  file_id UUID,
+  sent_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_messages_conversation ON messages(conversation_id, sent_at DESC);
+
+-- Files: encrypted before upload, metadata stored here
+CREATE TABLE IF NOT EXISTS files (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  owner_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  -- Encrypted filename (client-side)
+  encrypted_name TEXT NOT NULL,
+  -- MIME type (not sensitive, kept plaintext for serving)
+  mime_type TEXT NOT NULL DEFAULT 'application/octet-stream',
+  size_bytes BIGINT NOT NULL,
+  -- Path on disk (opaque UUID-based, not guessable)
+  storage_path TEXT NOT NULL UNIQUE,
+  -- AES-GCM file encryption key, encrypted with owner's master key
+  encrypted_file_key TEXT NOT NULL,
+  -- Optional: shared with a conversation
+  conversation_id UUID REFERENCES conversations(id) ON DELETE SET NULL,
+  uploaded_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  -- Soft delete
+  deleted_at TIMESTAMPTZ
+);
+
+CREATE INDEX IF NOT EXISTS idx_files_owner ON files(owner_id) WHERE deleted_at IS NULL;
+
+-- Public keys for E2E key exchange
+CREATE TABLE IF NOT EXISTS user_keys (
+  user_id UUID PRIMARY KEY REFERENCES users(id) ON DELETE CASCADE,
+  -- X25519 public key for ECDH (base64)
+  public_key TEXT NOT NULL,
+  -- Signed with user's identity key so others can verify
+  key_signature TEXT NOT NULL,
+  updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);