feat: add CSRF protection, SSRF prevention, and login rate limiting (#42)

## Security Hardening

This PR implements three security hardening issues:

### CSRF Protection (closes #35)

- Session-based CSRF tokens with cryptographically random 256-bit 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 webhook POSTs) and `/api` (stateless API)

### SSRF Prevention (closes #36)

- `ValidateTargetURL()` blocks private/reserved IP ranges at target creation time
- 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` in the delivery engine for defense-in-depth (prevents DNS rebinding attacks)
- Only `http` and `https` schemes allowed

### Login Rate Limiting (closes #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 every 5 minutes
- `X-Forwarded-For` and `X-Real-IP` header support for reverse proxies

### Files Changed

**New files:**
- `internal/middleware/csrf.go` + tests — CSRF middleware
- `internal/middleware/ratelimit.go` + tests — Login rate limiter
- `internal/delivery/ssrf.go` + tests — SSRF validation + safe transport

**Modified files:**
- `internal/server/routes.go` — Wire CSRF and rate limit middleware
- `internal/handlers/handlers.go` — Inject CSRF token into template data
- `internal/handlers/source_management.go` — SSRF validation on target creation
- `internal/delivery/engine.go` — SSRF-safe HTTP transport for production
- All form templates — Added hidden `csrf_token` fields
- `README.md` — Updated Security section and TODO checklist

`docker build .` passes (lint + tests + build).

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Co-authored-by: clawbot <clawbot@eeqj.de>
Co-authored-by: Jeffrey Paul <sneak@noreply.example.org>
Reviewed-on: #42
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/middleware/middleware_test.go

@@ -40,7 +40,7 @@ func testMiddleware(t *testing.T, env string) (*Middleware, *session.Session) {
 		SameSite: http.SameSiteLaxMode,
 	}
 
-	sessManager := newTestSession(t, store, cfg, log)
+	sessManager := newTestSession(t, store, cfg, log, key)
 
 	m := &Middleware{
 		log: log,
@@ -55,9 +55,9 @@ func testMiddleware(t *testing.T, env string) (*Middleware, *session.Session) {
 
 // newTestSession creates a session.Session with a pre-configured cookie store
 // for testing. This avoids needing the fx lifecycle and database.
-func newTestSession(t *testing.T, store *sessions.CookieStore, cfg *config.Config, log *slog.Logger) *session.Session {
+func newTestSession(t *testing.T, store *sessions.CookieStore, cfg *config.Config, log *slog.Logger, key []byte) *session.Session {
 	t.Helper()
-	return session.NewForTest(store, cfg, log)
+	return session.NewForTest(store, cfg, log, key)
 }
 
 // --- Logging Middleware Tests ---
@@ -326,7 +326,7 @@ func TestMetricsAuth_ValidCredentials(t *testing.T) {
 	store := sessions.NewCookieStore(key)
 	store.Options = &sessions.Options{Path: "/", MaxAge: 86400}
 
-	sessManager := session.NewForTest(store, cfg, log)
+	sessManager := session.NewForTest(store, cfg, log, key)
 
 	m := &Middleware{
 		log: log,
@@ -366,7 +366,7 @@ func TestMetricsAuth_InvalidCredentials(t *testing.T) {
 	store := sessions.NewCookieStore(key)
 	store.Options = &sessions.Options{Path: "/", MaxAge: 86400}
 
-	sessManager := session.NewForTest(store, cfg, log)
+	sessManager := session.NewForTest(store, cfg, log, key)
 
 	m := &Middleware{
 		log: log,
@@ -406,7 +406,7 @@ func TestMetricsAuth_NoCredentials(t *testing.T) {
 	store := sessions.NewCookieStore(key)
 	store.Options = &sessions.Options{Path: "/", MaxAge: 86400}
 
-	sessManager := session.NewForTest(store, cfg, log)
+	sessManager := session.NewForTest(store, cfg, log, key)
 
 	m := &Middleware{
 		log: log,