feat: add feed given event handling

- Add FEED_GIVEN to FeedInventoryProjection with apply/revert
- Add give_feed method to FeedService
- Block feed given if no purchase exists <= ts_utc
- Validate feed type and location existence
- 13 new tests for give_feed functionality

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

PLAN.md

@@ -153,12 +153,12 @@ Check off items as completed. Each phase builds on the previous.
 - [x] **Commit checkpoint** (5c10a75)
 
 ### Step 4.2: Feed Given Event
-- [ ] Implement apply_feed_given (increment given_kg, decrement balance)
-- [ ] Implement revert_feed_given
-- [ ] Add give_feed to services/feed.py
-- [ ] Block if no purchase <= ts_utc
-- [ ] Write tests: give updates inventory, blocked without purchase, revert
-- [ ] **Commit checkpoint**
+- [x] Implement apply_feed_given (increment given_kg, decrement balance)
+- [x] Implement revert_feed_given
+- [x] Add give_feed to services/feed.py
+- [x] Block if no purchase <= ts_utc
+- [x] Write tests: give updates inventory, blocked without purchase, revert
+- [x] **Commit checkpoint** (2eb0ca7)
 
 ### Step 4.3: Product Collection Event
 - [ ] Create `projections/products.py` for ProductCollected