feat: Phase 1 — reliability & completeness improvements

- httpx singleton: replace per-call AsyncClient with module-level singleton
  using keepalive pool (configurable via env vars). Close client on shutdown.

- Scraper pagination: multi-page fetching (default 2 pages/60 ads, configurable
  via SCRAPE_MAX_PAGES). Cursor-based early-stop to skip stale pages. Tracks
  ads_cursor per keyword for cross-cycle deduplication.

- Notification retry queue: persistent notification_queue table with exponential
  backoff (max 5 attempts → dead). Failed Telegram notifications are enqueued
  instead of silently dropped. Queue processed at start of each scheduler cycle
  (50 items max limit). /stats endpoint reports pending/dead counts.

worker/src/migrations/02-pagination-cursor.sql

@@ -0,0 +1,5 @@
+-- Add ads_cursor column to keywords for pagination cursor tracking
+
+ALTER TABLE keywords ADD COLUMN IF NOT EXISTS ads_cursor timestamptz;
+COMMENT ON COLUMN keywords.ads_cursor IS 
+    'Timestamp of oldest ad processed in last cycle, for pagination cursor';

worker/src/migrations/03-notification-queue.sql

@@ -0,0 +1,26 @@
+-- Notification retry queue table
+
+CREATE TYPE notification_type AS ENUM ('new', 'drop');
+CREATE TYPE notification_status AS ENUM ('pending', 'sent', 'failed', 'dead');
+
+CREATE TABLE IF NOT EXISTS notification_queue (
+    id            uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+    ad_id         uuid NOT NULL REFERENCES ads(id) ON DELETE SET NULL,
+    telegram_id   text NOT NULL,
+    message_text  text NOT NULL,
+    type          notification_type NOT NULL,
+    attempts      int NOT NULL DEFAULT 0,
+    max_attempts  int NOT NULL DEFAULT 5,
+    last_error    text,
+    status        notification_status NOT NULL DEFAULT 'pending',
+    created_at    timestamptz NOT NULL DEFAULT now(),
+    updated_at    timestamptz NOT NULL DEFAULT now()
+);
+
+-- Index for efficient queue polling
+CREATE INDEX IF NOT EXISTS idx_notif_queue_poll 
+    ON notification_queue(status, attempts, updated_at)
+    WHERE status IN ('pending', 'failed');
+
+COMMENT ON TABLE notification_queue IS 
+    'Persistent retry queue for failed Telegram notifications';