fix(#338): Bind CSRF token to user session with HMAC

- Token now includes HMAC binding to session ID
- Validates session binding on verification
- Adds CSRF_SECRET configuration requirement
- Requires authentication for CSRF token endpoint
- 51 new tests covering session binding security

Security: CSRF tokens are now cryptographically tied to user sessions,
preventing token reuse across sessions and mitigating session fixation
attacks.

Token format: {random_part}:{hmac(random_part + user_id, secret)}

Refs #338

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/api/.env.example

@@ -12,6 +12,12 @@ INSTANCE_URL=http://localhost:3000
 # Generate with: node -e "console.log(require('crypto').randomBytes(32).toString('hex'))"
 ENCRYPTION_KEY=0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef
 
+# CSRF Protection (Required in production)
+# Secret key for HMAC binding CSRF tokens to user sessions
+# Generate with: node -e "console.log(require('crypto').randomBytes(32).toString('hex'))"
+# In development, a random key is generated if not set
+CSRF_SECRET=fedcba9876543210fedcba9876543210fedcba9876543210fedcba9876543210
+
 # OpenTelemetry Configuration
 # Enable/disable OpenTelemetry tracing (default: true)
 OTEL_ENABLED=true