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/ratelimit.go

@@ -0,0 +1,48 @@
+package middleware
+
+import (
+	"net/http"
+	"time"
+
+	"github.com/go-chi/httprate"
+)
+
+const (
+	// loginRateLimit is the maximum number of login attempts per interval.
+	loginRateLimit = 5
+
+	// loginRateInterval is the time window for the rate limit.
+	loginRateInterval = 1 * time.Minute
+)
+
+// LoginRateLimit returns middleware that enforces per-IP rate limiting
+// on login attempts using go-chi/httprate. Only POST requests are
+// rate-limited; GET requests (rendering the login form) pass through
+// unaffected. When the rate limit is exceeded, a 429 Too Many Requests
+// response is returned. IP extraction honours X-Forwarded-For,
+// X-Real-IP, and True-Client-IP headers for reverse-proxy setups.
+func (m *Middleware) LoginRateLimit() func(http.Handler) http.Handler {
+	limiter := httprate.Limit(
+		loginRateLimit,
+		loginRateInterval,
+		httprate.WithKeyFuncs(httprate.KeyByRealIP),
+		httprate.WithLimitHandler(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			m.log.Warn("login rate limit exceeded",
+				"path", r.URL.Path,
+			)
+			http.Error(w, "Too many login attempts. Please try again later.", http.StatusTooManyRequests)
+		})),
+	)
+
+	return func(next http.Handler) http.Handler {
+		limited := limiter(next)
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			// Only rate-limit POST requests (actual login attempts)
+			if r.Method != http.MethodPost {
+				next.ServeHTTP(w, r)
+				return
+			}
+			limited.ServeHTTP(w, r)
+		})
+	}
+}