fix(backend): harden security and speed up feeds and article API

Security:
- WebSocket events are routed to their owner only (no cross-user leak);
  hub close is idempotent (fixes double-close panic), adds ping/pong and
  write deadlines.
- Session tokens stored as SHA-256 (migration 008 keeps sessions valid);
  single-query auth middleware puts the user in the request context.
- Client IP only trusts X-Forwarded-For from TRUSTED_PROXIES; rate limiter
  map is bounded; per-user limit on AI summaries.
- Argon2id at OWASP minimum with a concurrency cap; constant-time login
  for unknown emails; atomic first-admin bootstrap; REGISTRATION_ENABLED.
- CSP/HSTS/COOP headers, same-origin guard on mutations, body size limits,
  wider SSRF denylist, bounded feed/page/AI response reads, generic errors.
- Upgrade chi, pgx, x/net, x/text, x/crypto (known CVEs); commit go.sum.

Performance:
- List endpoints return a plain-text excerpt and reading time instead of
  full HTML; content is sanitized once at ingest (legacy rows backfilled).
- Keyset pagination on (sort_at, id) with matching partial indexes;
  redundant indexes dropped (migration 007).
- Fetcher: bounded worker pool, conditional GET (ETag/Last-Modified),
  exponential backoff, dedupe before insert, column-safe truncation,
  retention-aware ingest, per-user refresh coalescing.
- Read/favorite/read-all are single ownership-scoped statements.
- gzip compression, immutable caching for hashed assets, path-safe SPA
  handler, server timeouts; expired sessions purged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/handler/ws.go

@@ -1,7 +1,6 @@
 package handler
 
 import (
-	"log"
 	"net/http"
 
 	"github.com/michael/flowreader/internal/service"
@@ -22,20 +21,12 @@ func NewWSHandler(hub *ws.Hub, authService *service.AuthService) *WSHandler {
 	}
 }
 
-// Connect handles WebSocket initiation.
+// Connect handles WebSocket initiation (behind RequireAuth).
 func (h *WSHandler) Connect(w http.ResponseWriter, r *http.Request) {
-	cookie, err := r.Cookie("session_id")
-	if err != nil {
+	user := currentUser(r)
+	if user == nil {
 		http.Error(w, "Unauthorized", http.StatusUnauthorized)
 		return
 	}
-
-	user, err := h.authService.GetUserByToken(cookie.Value)
-	if err != nil || user == nil {
-		http.Error(w, "Unauthorized", http.StatusUnauthorized)
-		return
-	}
-
-	log.Printf("Setting up WS for user %s", user.ID)
 	h.hub.ServeWS(user.ID, w, r)
 }