feat(discovery): add discovered_markets queue table migration

backend/migrations/000012_discovered_markets.down.sql

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+DROP TABLE IF EXISTS discovered_markets;

backend/migrations/000012_discovered_markets.up.sql

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+CREATE TABLE discovered_markets (
+  id                  uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  bucket_id           uuid NOT NULL REFERENCES discovery_buckets(id) ON DELETE CASCADE,
+  markt_name          text NOT NULL,
+  stadt               text NOT NULL,
+  bundesland          text,
+  land                text NOT NULL,
+  start_datum         date,
+  end_datum           date,
+  website             text,
+  quellen             text[] NOT NULL DEFAULT '{}',
+  extraktion          text,
+  hinweis             text,
+  name_normalized     text NOT NULL,
+  matched_series_id   uuid REFERENCES market_series(id) ON DELETE SET NULL,
+  status              text NOT NULL DEFAULT 'pending'
+    CHECK (status IN ('pending', 'accepted', 'rejected')),
+  discovered_at       timestamptz NOT NULL DEFAULT now(),
+  reviewed_at         timestamptz,
+  reviewed_by         uuid REFERENCES users(id) ON DELETE SET NULL,
+  created_edition_id  uuid REFERENCES market_editions(id) ON DELETE SET NULL
+);
+
+CREATE INDEX idx_discovered_markets_pending
+  ON discovered_markets (status, discovered_at)
+  WHERE status = 'pending';
+
+CREATE INDEX idx_discovered_markets_dedup
+  ON discovered_markets (name_normalized, stadt, start_datum)
+  WHERE status = 'pending';