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/utils/sanitizer.go

@@ -6,16 +6,21 @@ import (
 )
 
 // ContentSanitizer handles HTML sanitization for articles.
+// A bluemonday policy is safe for concurrent use once built.
 type ContentSanitizer struct {
 	policy *bluemonday.Policy
 }
 
 // NewContentSanitizer creates a new sanitizer with a "UGCPolicy" (safe for user-generated content).
 func NewContentSanitizer() *ContentSanitizer {
-	// Using UGCPolicy allows common tags (b, i, p, img, etc.) but strips dangerous ones.
-	return &ContentSanitizer{
-		policy: bluemonday.UGCPolicy(),
-	}
+	// UGCPolicy allows common tags (b, i, p, img, figure, table…) but strips
+	// scripts, styles, event handlers and non-http(s)/mailto links.
+	policy := bluemonday.UGCPolicy()
+	// Article links open in a new tab instead of navigating away from the
+	// reader (rel="noopener noreferrer nofollow" is added automatically).
+	policy.AddTargetBlankToFullyQualifiedLinks(true)
+	policy.RequireNoReferrerOnFullyQualifiedLinks(true)
+	return &ContentSanitizer{policy: policy}
 }
 
 // Sanitize cleans the HTML content.