feat: add feed inventory schema and purchase service

Implement FeedPurchased event handling:
- Add migration for feed_inventory table
- Create FeedInventoryProjection to track purchases
- Create FeedService with purchase_feed method
- Calculate price_per_kg_cents from bag details

Purchases accumulate in inventory with:
- purchased_kg, given_kg, balance_kg tracking
- Last purchase price stored in cents
- Timestamps for last purchase/given

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

migrations/0006-feed-inventory.sql

@@ -0,0 +1,18 @@
+-- ABOUTME: Migration to create feed_inventory table.
+-- ABOUTME: Tracks feed purchases, amounts given, and current balance.
+
+-- Feed inventory tracks global feed levels per type
+-- Stores amounts in kg, prices in cents
+CREATE TABLE feed_inventory (
+    feed_type_code TEXT PRIMARY KEY REFERENCES feed_types(code),
+    purchased_kg INTEGER NOT NULL DEFAULT 0,
+    given_kg INTEGER NOT NULL DEFAULT 0,
+    balance_kg INTEGER NOT NULL DEFAULT 0,
+    last_purchase_price_per_kg_cents INTEGER,
+    last_purchase_at_utc INTEGER,
+    last_given_at_utc INTEGER,
+    updated_at_utc INTEGER NOT NULL
+);
+
+-- Index for finding when last purchase/given occurred
+CREATE INDEX idx_feed_inventory_last_on ON feed_inventory(last_purchase_at_utc, last_given_at_utc);