feat(discovery): add rejected_discoveries sticky-reject table migration

backend/migrations/000013_rejected_discoveries.down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS rejected_discoveries;

backend/migrations/000013_rejected_discoveries.up.sql

@@ -0,0 +1,13 @@
+CREATE TABLE rejected_discoveries (
+  id              uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  name_normalized text NOT NULL,
+  stadt           text NOT NULL,
+  year            int  NOT NULL,
+  rejected_at     timestamptz NOT NULL DEFAULT now(),
+  rejected_by     uuid REFERENCES users(id) ON DELETE SET NULL,
+  reason          text,
+  UNIQUE (name_normalized, stadt, year)
+);
+
+CREATE INDEX idx_rejected_discoveries_lookup
+  ON rejected_discoveries (name_normalized, stadt, year);