Add Go backend unit and handler tests; wire test step into CI

- Introduce store interfaces (TrackpointStorer, StopStorer, SuggestionStorer)
  in internal/db/interfaces.go so handlers can be tested without a real DB.
- Refactor HandleSingleTrackpoint, HandleBatchTrackpoints, HandleListDays,
  HandleListTrackpoints, HandleListStops, HandleListSuggestions to accept
  the new interfaces instead of concrete *db.*Store pointers (no behaviour
  change; concrete types satisfy the interfaces implicitly).
- internal/api/ingest_test.go: 13 handler tests covering happy path,
  invalid JSON, invalid timestamp, missing event_id/device_id, out-of-range
  lat/lon, empty/oversized batch, store errors, and idempotency (single + batch).
- internal/api/query_test.go: 14 handler tests covering missing query params
  (400) and empty-result-is-array guarantees for all four query endpoints.
- internal/auth/auth_test.go: 5 unit tests for HashPassword / VerifyPassword
  (correct password, wrong password, empty password, malformed hash, salt
  uniqueness).
- internal/db/trackpoints_test.go: 6 unit tests for the validateTrackpoint
  helper (happy path, missing fields, coordinate bounds, invalid source).
- .gitea/workflows/deploy.yml: add "Test" step (go test ./...) before
  "Build & Deploy" so a failing test aborts the deployment.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/deploy.yml

@@ -23,6 +23,9 @@ jobs:
       - name: Write .env
         run: printf 'DB_PASSWORD=%s\n' '${{ secrets.DB_PASSWORD }}' > ${{ vars.DEPLOY_DIR }}/.env
 
+      - name: Test
+        run: cd ${{ vars.DEPLOY_DIR }}/backend && go test ./...
+
       - name: Build & Deploy
         run: docker compose -f ${{ vars.DEPLOY_DIR }}/docker-compose.yml up --build -d