Wrap /user/{username} in RequireAuth middleware (closes #60) (#71)

Enforces authentication for the `/user/{username}` route group at the middleware layer, matching every other authenticated route group.

## Changes

- **`internal/server/routes.go`** (`setupUserRoutes`): added `r.Use(s.mw.RequireAuth())` immediately after the existing `r.Use(s.mw.CSRF())` on the `/user/{username}` group, so auth is enforced by design (CSRF first, then RequireAuth) — consistent with `/sources` and `/source/{sourceID}`.
- **`internal/handlers/profile.go`** (`HandleProfile`): removed the now-dead unauthenticated-redirect branch (RequireAuth guarantees an authenticated session before the handler runs). The handler still reads the username and user id from the session for the own-profile-only check; a request for another user's profile still returns 403. The session-retrieval error is now handled as a 500.

## Tests (`internal/handlers/profile_test.go`)

- own profile returns 200
- another user's profile returns 403
- an unauthenticated request to `/user/{username}` is redirected to `/pages/login` at the middleware layer and never reaches the endpoint handler (routing-level test replicating the CSRF + RequireAuth chain)

## Validation

`docker build .` (fmt-check, lint, test, build) passes.

Closes #60

Co-authored-by: sneak <sneak@sneak.berlin>
Co-authored-by: Jeffrey Paul <sneak@noreply.example.org>
Reviewed-on: #71
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/handlers/profile.go

@@ -17,11 +17,13 @@ func (h *Handlers) HandleProfile() http.HandlerFunc {
 			return
 		}
 
-		// Get session
+		// Get session. RequireAuth middleware guarantees an
+		// authenticated session before this handler runs, so we
+		// only need to guard against an unexpected retrieval error.
 		sess, err := h.session.Get(r)
-		if err != nil || !h.session.IsAuthenticated(sess) {
-			// Redirect to login if not authenticated
-			http.Redirect(w, r, "/pages/login", http.StatusSeeOther)
+		if err != nil {
+			h.log.Error("failed to get session", "error", err)
+			http.Error(w, "Internal server error", http.StatusInternalServerError)
 
 			return
 		}