feat: add episode availability cache and provider mapping tables

internal/database/migrations/017_add_episode_availability.sql

@@ -0,0 +1,27 @@
+-- +goose Up
+CREATE TABLE IF NOT EXISTS episode_availability_cache (
+    anime_id INTEGER PRIMARY KEY,
+    data TEXT NOT NULL,
+    next_refresh_at DATETIME,
+    retry_until_at DATETIME,
+    last_attempt_at DATETIME,
+    last_success_at DATETIME,
+    failure_count INTEGER NOT NULL DEFAULT 0,
+    last_error TEXT NOT NULL DEFAULT '',
+    updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE TABLE IF NOT EXISTS episode_provider_mapping (
+    anime_id INTEGER NOT NULL,
+    provider TEXT NOT NULL,
+    provider_show_id TEXT NOT NULL,
+    failed_until DATETIME,
+    last_error TEXT NOT NULL DEFAULT '',
+    updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    PRIMARY KEY (anime_id, provider)
+);
+
+CREATE INDEX IF NOT EXISTS idx_episode_availability_next_refresh
+ON episode_availability_cache(next_refresh_at);
+
+-- +goose Down