feat: persist rate overrides to DB when applying prices in AvailabilityPage

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/migrations/022_rate_overrides.sql

@@ -0,0 +1,12 @@
+CREATE TABLE IF NOT EXISTS rate_overrides (
+  hotel_id       UUID    NOT NULL REFERENCES hotels(id) ON DELETE CASCADE,
+  category_id    TEXT    NOT NULL,
+  date           DATE    NOT NULL,
+  price          INTEGER NOT NULL,
+  extra_person   INTEGER NOT NULL DEFAULT 0,
+  min_nights     INTEGER NOT NULL DEFAULT 1,
+  channel_prices JSONB   NOT NULL DEFAULT '{}',
+  closed         BOOLEAN NOT NULL DEFAULT false,
+  updated_at     TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  PRIMARY KEY (hotel_id, category_id, date)
+);