feat: add ebay_categories migration for LLM query builder category cache

app/db/migrations/011_ebay_categories.sql

@@ -0,0 +1,16 @@
+-- app/db/migrations/011_ebay_categories.sql
+-- eBay category leaf node cache. Refreshed weekly via EbayCategoryCache.refresh().
+-- Seeded with a small bootstrap table when no eBay API credentials are configured.
+-- MIT License
+
+CREATE TABLE IF NOT EXISTS ebay_categories (
+    id           INTEGER PRIMARY KEY AUTOINCREMENT,
+    category_id  TEXT NOT NULL UNIQUE,
+    name         TEXT NOT NULL,
+    full_path    TEXT NOT NULL,   -- "Consumer Electronics > ... > Leaf Name"
+    is_leaf      INTEGER NOT NULL DEFAULT 1,  -- SQLite stores bool as int
+    refreshed_at TEXT NOT NULL    -- ISO8601 timestamp
+);
+
+CREATE INDEX IF NOT EXISTS idx_ebay_cat_name
+    ON ebay_categories (name);