feat: add API unit tests

Add Bun test suite covering:
- Auth: register, login, duplicate email, invalid password
- Events: create, location, validation, unauthorized
- Journals: get, generate, delete, list with pagination
- Settings: get, update, clear prompt, change password
- Export/Import: export data, events with location
- Search: search events, no matches, cross-date
- Days: list days, get day details, empty day

AGENTS.md

@@ -18,6 +18,7 @@ Self-hosted journaling app where users capture events throughout the day and AI
   /src
     index.ts               # Main API routes, journal generation logic
     /services/ai           # AI provider implementations (Groq, OpenAI, Anthropic, Ollama, LMStudio)
+    /__tests__/            # API unit tests (Bun test)
 
 /frontend
   /src
@@ -191,6 +192,13 @@ bunx prisma migrate dev --name migration_name
 docker compose build && docker compose up -d
 ```
 
+### Running Tests
+```bash
+cd backend
+bun run test:server
+```
+Tests require the server running. The test script starts the server, runs tests, then stops it.
+
 ## Version History
 - 0.1.0: Automatic geolocation capture, Starlight documentation site
 - 0.0.6: Automatic geolocation capture on event creation, reverse geocoding to place names