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/handlers/handlers.go

@@ -13,6 +13,7 @@ import (
 	"sneak.berlin/go/webhooker/internal/globals"
 	"sneak.berlin/go/webhooker/internal/healthcheck"
 	"sneak.berlin/go/webhooker/internal/logger"
+	"sneak.berlin/go/webhooker/internal/middleware"
 	"sneak.berlin/go/webhooker/internal/session"
 	"sneak.berlin/go/webhooker/templates"
 )
@@ -128,9 +129,13 @@ func (s *Handlers) renderTemplate(w http.ResponseWriter, r *http.Request, pageTe
 		}
 	}
 
-	// If data is a map, merge user info into it
+	// Get CSRF token from request context (set by CSRF middleware)
+	csrfToken := middleware.CSRFToken(r)
+
+	// If data is a map, merge user info and CSRF token into it
 	if m, ok := data.(map[string]interface{}); ok {
 		m["User"] = userInfo
+		m["CSRFToken"] = csrfToken
 		if err := tmpl.Execute(w, m); err != nil {
 			s.log.Error("failed to execute template", "error", err)
 			http.Error(w, "Internal server error", http.StatusInternalServerError)
@@ -140,13 +145,15 @@ func (s *Handlers) renderTemplate(w http.ResponseWriter, r *http.Request, pageTe
 
 	// Wrap data with base template data
 	type templateDataWrapper struct {
-		User *UserInfo
-		Data interface{}
+		User      *UserInfo
+		CSRFToken string
+		Data      interface{}
 	}
 
 	wrapper := templateDataWrapper{
-		User: userInfo,
-		Data: data,
+		User:      userInfo,
+		CSRFToken: csrfToken,
+		Data:      data,
 	}
 
 	if err := tmpl.Execute(w, wrapper); err != nil {