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.

tests/test_indexer.py

@@ -21,6 +21,9 @@ from indexer import (
     parse_email_message,
     get_recent_keys,
     METADATA_COLLECTION,
+    EMBEDDING_MODEL_NAME,
+    BATCH_SIZE,
+    EMBEDDING_BATCH_SIZE,
 )
 
 
@@ -227,3 +230,25 @@ class TestGetRecentKeys:
         assert key.split(":")[0] in keys
         for k in keys:
             assert ":" not in k
+
+
+# ---------------------------------------------------------------------------
+# Regression: module-level constants consumed by main()
+# ---------------------------------------------------------------------------
+
+
+def test_indexer_module_constants_are_defined():
+    """Regression (e32806a): the "make indexer importable without env" refactor
+    dropped these three module-level definitions while main() kept using them,
+    so every indexer run died with NameError before touching Qdrant."""
+    assert isinstance(EMBEDDING_MODEL_NAME, str) and EMBEDDING_MODEL_NAME
+    assert isinstance(BATCH_SIZE, int) and BATCH_SIZE > 0
+    assert isinstance(EMBEDDING_BATCH_SIZE, int) and EMBEDDING_BATCH_SIZE > 0
+
+
+def test_embedding_model_is_the_same_as_the_server_one():
+    """Indexer and server must embed with the same model, otherwise the vectors
+    stored by the indexer are not comparable with the query vectors."""
+    from server import EMBEDDING_MODEL_NAME as server_embedding_model
+
+    assert EMBEDDING_MODEL_NAME == server_embedding_model