feat: add CSRF protection, SSRF prevention, and login rate limiting

Security hardening implementing three issues:

CSRF Protection (#35):
- Session-based CSRF tokens with cryptographically random generation
- Constant-time token comparison to prevent timing attacks
- CSRF middleware applied to /pages, /sources, /source, and /user routes
- Hidden csrf_token field added to all 12+ POST forms in templates
- Excluded from /webhook (inbound) and /api (stateless) routes

SSRF Prevention (#36):
- ValidateTargetURL blocks private/reserved IP ranges at target creation
- Blocked ranges: 127.0.0.0/8, 10.0.0.0/8, 172.16.0.0/12,
  192.168.0.0/16, 169.254.0.0/16, ::1, fc00::/7, fe80::/10, plus
  multicast, reserved, test-net, and CGN ranges
- SSRF-safe HTTP transport with custom DialContext for defense-in-depth
  at delivery time (prevents DNS rebinding attacks)
- Only http/https schemes allowed

Login Rate Limiting (#37):
- Per-IP rate limiter using golang.org/x/time/rate
- 5 attempts per minute per IP on POST /pages/login
- GET requests (form rendering) pass through unaffected
- Automatic cleanup of stale per-IP limiter entries
- X-Forwarded-For and X-Real-IP header support for reverse proxies

Closes #35, closes #36, closes #37

internal/delivery/engine.go

@@ -146,7 +146,8 @@ func New(lc fx.Lifecycle, params EngineParams) *Engine {
 		dbManager: params.DBManager,
 		log:       params.Logger.Get(),
 		client: &http.Client{
-			Timeout: httpClientTimeout,
+			Timeout:   httpClientTimeout,
+			Transport: NewSSRFSafeTransport(),
 		},
 		deliveryCh: make(chan DeliveryTask, deliveryChannelSize),
 		retryCh:    make(chan DeliveryTask, retryChannelSize),