feat: implement user defaults persistence (Step 9.3)

Add user_defaults table and repository for persisting form defaults
across sessions. Feed and egg forms now load/save user preferences.

Changes:
- Add migration 0009-user-defaults.sql with table schema
- Add UserDefault model and UserDefaultsRepository
- Integrate defaults into feed route (location, feed_type, amount)
- Integrate defaults into egg route (location)
- Add repository unit tests and route integration tests

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

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

PLAN.md

@@ -359,12 +359,12 @@ Check off items as completed. Each phase builds on the previous.
 - [x] Write tests: sale creates event, unit price calculated
 - [x] **Commit checkpoint**: 0eef3ed
 
-### Step 9.3: User Defaults
-- [ ] Create migration for user_defaults table
-- [ ] Create `repositories/user_defaults.py`
-- [ ] Integrate defaults into form rendering
-- [ ] Write tests: defaults saved and loaded
-- [ ] **Commit checkpoint**
+### Step 9.3: User Defaults ✅
+- [x] Create migration for user_defaults table
+- [x] Create `repositories/user_defaults.py`
+- [x] Integrate defaults into form rendering
+- [x] Write tests: defaults saved and loaded
+- [x] **Commit checkpoint**
 
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