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/worker/fetcher.go

@@ -19,7 +19,8 @@ type FeedFetcher struct {
 	wg           sync.WaitGroup
 }
 
-// NewFeedFetcher creates a new feed fetcher worker.
+// NewFeedFetcher creates a new feed fetcher worker. The interval is how often
+// due feeds are looked for; each feed carries its own next_fetch_at.
 func NewFeedFetcher(fetchService *service.FetchService, interval time.Duration, concurrency int) *FeedFetcher {
 	return &FeedFetcher{
 		fetchService: fetchService,
@@ -66,7 +67,7 @@ func (f *FeedFetcher) fetch() {
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Minute)
 	defer cancel()
 
-	count, err := f.fetchService.FetchAllPending(ctx, f.concurrency)
+	count, err := f.fetchService.FetchAllPending(ctx)
 	if err != nil {
 		log.Printf("Feed fetch error: %v", err)
 		return