security: remediate all P0-P2 audit findings (12 tasks)

P0 fixes:
- WebSocket origin checking (reject untrusted origins)
- WS session token moved from URL query param to first message frame
- WebAuthn login cookie now uses Secure flag via shared SetSessionCookie
- PostgreSQL sslmode configurable via POSTGRES_SSLMODE env (default: require)
- Fixed DatabaseDSN to use real password instead of masked placeholder

P1 fixes:
- Per-IP rate limiting middleware (auth: 5 req/s, invites: 2 req/s)
- Security headers on all responses (CSP, X-Frame-Options, nosniff, etc.)
- WS broadcasts scoped to server members (prevents cross-server data leak)
- Webhook tokens stored as SHA-256 hashes (not plaintext)

P2 fixes:
- CSRF protection via Origin header validation on state-changing requests
- MANAGE_CHANNELS permission enforced on channel update/delete
- Upload validation: 25MB limit, extension allowlist, server-side MIME check
- File serve: path traversal protection + Content-Disposition: attachment

Files: 23 changed, +499/-100. Builds clean (go build, go vet, npm build).
Zero CVEs (govulncheck, npm audit).

web/src/stores/ws.ts

@@ -70,7 +70,7 @@ export const useWebSocketStore = create<WebSocketState>((set, get) => ({
       return;
     }
 
-    const socket = new WebSocket(`${getWsHost()}/ws?token=${encodeURIComponent(token)}`);
+    const socket = new WebSocket(`${getWsHost()}/ws`);
 
     let reconnectDelay = 1000;
     const maxReconnectDelay = 30000;
@@ -89,8 +89,9 @@ export const useWebSocketStore = create<WebSocketState>((set, get) => ({
     };
 
     socket.onopen = () => {
-      set({ connected: true });
-      reconnectDelay = 1000;
+      // Send auth token as the first message frame instead of a URL param.
+      // This keeps the token out of server logs, browser history, and Referer headers.
+      socket.send(JSON.stringify({ token }));
     };
 
     socket.onmessage = (event) => {
@@ -101,6 +102,20 @@ export const useWebSocketStore = create<WebSocketState>((set, get) => ({
         return;
       }
 
+      // Server sends {"type":"ready"} after successful auth.
+      if (data.type === 'ready') {
+        set({ connected: true });
+        reconnectDelay = 1000;
+        return;
+      }
+
+      if (data.type === 'error') {
+        // Auth failed or server sent an error. Close and don't reconnect.
+        console.error('ws error:', data);
+        socket.close();
+        return;
+      }
+
       if (data.type === 'ping') {
         get().send({ type: 'pong', payload: {} });
         return;