security: add headers middleware, session regeneration, and body size limits (#41)

## Summary

This PR implements three security hardening measures:

### Security Headers Middleware (closes #34)

Adds a `SecurityHeaders()` middleware applied globally to all routes. Every response now includes:
- `Strict-Transport-Security: max-age=63072000; includeSubDomains; preload`
- `X-Content-Type-Options: nosniff`
- `X-Frame-Options: DENY`
- `Content-Security-Policy: default-src 'self'; script-src 'self' 'unsafe-inline'; style-src 'self' 'unsafe-inline'`
- `Referrer-Policy: strict-origin-when-cross-origin`
- `Permissions-Policy: camera=(), microphone=(), geolocation=()`

### Session Fixation Prevention (closes #38)

Adds a `Regenerate()` method to the session manager that destroys the old session and creates a new one with a fresh ID, copying all session values. Called after successful login to prevent session fixation attacks.

### Request Body Size Limits (closes #39)

Adds a `MaxBodySize()` middleware using `http.MaxBytesReader` to limit POST/PUT/PATCH request bodies to 1 MB. Applied to all form endpoints (`/pages`, `/sources`, `/source/*`).

## Files Changed

- `internal/middleware/middleware.go` — Added `SecurityHeaders()` and `MaxBodySize()` middleware
- `internal/session/session.go` — Added `Regenerate()` method for session fixation prevention
- `internal/handlers/auth.go` — Updated login handler to regenerate session after authentication
- `internal/server/routes.go` — Added SecurityHeaders globally, MaxBodySize to form route groups
- `README.md` — Documented new middleware in stack, updated Security section, moved items to completed TODO

closes #34, closes #38, closes #39

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #41
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/middleware/middleware.go

@@ -171,3 +171,35 @@ func (s *Middleware) MetricsAuth() func(http.Handler) http.Handler {
 		},
 	)
 }
+
+// SecurityHeaders returns middleware that sets production security headers
+// on every response: HSTS, X-Content-Type-Options, X-Frame-Options, CSP,
+// Referrer-Policy, and Permissions-Policy.
+func (s *Middleware) SecurityHeaders() func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.Header().Set("Strict-Transport-Security", "max-age=63072000; includeSubDomains; preload")
+			w.Header().Set("X-Content-Type-Options", "nosniff")
+			w.Header().Set("X-Frame-Options", "DENY")
+			w.Header().Set("Content-Security-Policy", "default-src 'self'; script-src 'self' 'unsafe-inline'; style-src 'self' 'unsafe-inline'")
+			w.Header().Set("Referrer-Policy", "strict-origin-when-cross-origin")
+			w.Header().Set("Permissions-Policy", "camera=(), microphone=(), geolocation=()")
+			next.ServeHTTP(w, r)
+		})
+	}
+}
+
+// MaxBodySize returns middleware that limits the request body size for POST
+// requests. If the body exceeds the given limit in bytes, the server returns
+// 413 Request Entity Too Large. This prevents clients from sending arbitrarily
+// large form bodies.
+func (s *Middleware) MaxBodySize(maxBytes int64) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			if r.Method == http.MethodPost || r.Method == http.MethodPut || r.Method == http.MethodPatch {
+				r.Body = http.MaxBytesReader(w, r.Body, maxBytes)
+			}
+			next.ServeHTTP(w, r)
+		})
+	}
+}