fix(indexer): restore module-level embedding/batch constants

The "make indexer importable without env" refactor (e32806a, merged via PR
#52 on 2026-10-09) removed EMBEDDING_MODEL_NAME, BATCH_SIZE and
EMBEDDING_BATCH_SIZE from the module scope while main() kept referencing
them, so every indexer run died before reaching Qdrant:

    File "/app/src/indexer.py", line 264, in main
        print(f"Loading embedding model: {EMBEDDING_MODEL_NAME}...")
    NameError: name 'EMBEDDING_MODEL_NAME' is not defined

CI never caught it because no test executes main(). Both *-indexer cronjobs
have been failing (BackoffLimitExceeded) since 2026-10-16, i.e. the first
run after the image pin moved to v0.0.22-latest.

- restore the three constants with their pre-refactor defaults
- cover them in tests/test_indexer.py and cross-check that the indexer
  embeds with the same model as server.py (vectors must stay comparable)
- add a ruff F821 gate to the unit job so undefined names fail CI

Fixes mcp-maildir-cabilloteu-indexer and mcp-maildir-gmail-indexer.

.gitea/workflows/main.yaml

@@ -22,7 +22,9 @@ jobs:
         with:
           python-version: "3.14"
       - name: Install dependencies
-        run: pip install -r requirements.txt pytest
+        run: pip install -r requirements.txt pytest ruff==0.17.1
+      - name: Lint (undefined names)
+        run: ruff check src/ tests/ --select F821
       - name: Run unit tests
         run: pytest tests/ -v
         env: