feat: complete Phase 4 combat/hunting system

- Add content migration (003_content.sql): zones, monsters, items, loot tables
- Add inventory migration (004_inventory.sql) and logs migration (006_logs.sql)
- Add Mulberry32 PRNG (src/utils/rng.js) for deterministic combat
- Add combat system (src/game/combat.js) with turn order, crit, mage bonus, 20-round cap
- Add rewards/loot system (src/game/rewards.js) and leveling (src/game/leveling.js)
- Add hunt endpoint (POST /api/hunt) with full combat flow in DB transaction
- Add hunt integration tests and unit tests for rewards/leveling/combat
- Fix session middleware to use memory store in tests (avoids cleanup hang)
- Fix huntService transaction (db.transaction() returns wrapper, must call separately)
- Add huntService to helpers.js cache invalidation for test isolation

src/db/migrations/004_inventory.sql

@@ -0,0 +1,16 @@
+-- 004_inventory.sql — Player inventory and equipment slots.
+
+CREATE TABLE IF NOT EXISTS inventory (
+  id            INTEGER PRIMARY KEY AUTOINCREMENT,
+  character_id  INTEGER NOT NULL REFERENCES characters(id) ON DELETE CASCADE,
+  item_id       INTEGER NOT NULL REFERENCES items(id),
+  quantity      INTEGER NOT NULL DEFAULT 1 CHECK (quantity >= 1),
+  equipped_slot TEXT CHECK (equipped_slot IN ('weapon', 'armor', 'trinket')),
+  acquired_at   INTEGER NOT NULL
+);
+
+CREATE INDEX IF NOT EXISTS idx_inventory_char ON inventory(character_id);
+
+-- One equipped item per slot per character (partial unique index).
+CREATE UNIQUE INDEX IF NOT EXISTS idx_inventory_equipped
+  ON inventory(character_id, equipped_slot) WHERE equipped_slot IS NOT NULL;