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>

migrations/007_perf_reading.up.sql

@@ -0,0 +1,38 @@
+-- Migration: 007_perf_reading
+-- Description: list-friendly derived columns, keyset-pagination indexes,
+-- conditional GET / backoff state for feeds, and removal of redundant indexes.
+
+-- Derived article columns. Filled at ingest; legacy rows are backfilled by the
+-- application at startup (word_count IS NULL marks a row as not yet processed).
+ALTER TABLE articles ADD COLUMN IF NOT EXISTS excerpt TEXT;
+ALTER TABLE articles ADD COLUMN IF NOT EXISTS word_count INTEGER;
+
+-- Non-null sort key so keyset pagination works on (sort_at, id).
+ALTER TABLE articles ADD COLUMN IF NOT EXISTS sort_at TIMESTAMPTZ
+    GENERATED ALWAYS AS (COALESCE(published_at, created_at)) STORED;
+
+CREATE INDEX IF NOT EXISTS idx_articles_feed_sort        ON articles (feed_id, sort_at DESC, id DESC);
+CREATE INDEX IF NOT EXISTS idx_articles_feed_unread_sort ON articles (feed_id, sort_at DESC, id DESC) WHERE NOT is_read;
+CREATE INDEX IF NOT EXISTS idx_articles_sort             ON articles (sort_at DESC, id DESC);
+CREATE INDEX IF NOT EXISTS idx_articles_unread_sort      ON articles (sort_at DESC, id DESC) WHERE NOT is_read;
+CREATE INDEX IF NOT EXISTS idx_articles_fav_sort         ON articles (sort_at DESC, id DESC) WHERE is_favorite;
+CREATE INDEX IF NOT EXISTS idx_articles_unread_feed      ON articles (feed_id) WHERE NOT is_read;
+CREATE INDEX IF NOT EXISTS idx_articles_cleanup          ON articles (created_at) WHERE NOT is_favorite;
+CREATE INDEX IF NOT EXISTS idx_articles_backfill         ON articles (id) WHERE word_count IS NULL;
+
+-- Redundant or unusable indexes: each one costs a write on every INSERT and
+-- every is_read UPDATE.
+DROP INDEX IF EXISTS idx_articles_feed_id;      -- prefix of UNIQUE(feed_id, guid)
+DROP INDEX IF EXISTS idx_articles_published_at; -- wrong NULLS order, unused
+DROP INDEX IF EXISTS idx_articles_is_read;      -- replaced by partial indexes
+DROP INDEX IF EXISTS idx_articles_is_favorite;  -- replaced by partial index
+DROP INDEX IF EXISTS idx_users_email;           -- duplicate of UNIQUE(email)
+DROP INDEX IF EXISTS idx_sessions_token;        -- duplicate of UNIQUE(token)
+DROP INDEX IF EXISTS idx_feeds_user_id;         -- prefix of UNIQUE(user_id, url)
+
+-- Feed fetch state: HTTP conditional GET + exponential backoff on errors.
+ALTER TABLE feeds ADD COLUMN IF NOT EXISTS etag TEXT;
+ALTER TABLE feeds ADD COLUMN IF NOT EXISTS last_modified TEXT;
+ALTER TABLE feeds ADD COLUMN IF NOT EXISTS error_count INTEGER NOT NULL DEFAULT 0;
+ALTER TABLE feeds ADD COLUMN IF NOT EXISTS next_fetch_at TIMESTAMPTZ;
+CREATE INDEX IF NOT EXISTS idx_feeds_next_fetch ON feeds (next_fetch_at NULLS FIRST);