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.

src/indexer.py

@@ -29,6 +29,13 @@ MAILDIR_FOLDERS = os.environ.get("MAILDIR_FOLDERS", "")
 QDRANT_URL = os.environ.get("QDRANT_URL", "")
 COLLECTION_NAME = os.environ.get("COLLECTION_NAME", "")
 
+# Embedding model and batching. EMBEDDING_MODEL_NAME MUST stay in sync with the
+# one used by server.py: indexer and server have to embed with the same model,
+# otherwise the vectors stored in Qdrant are not comparable with query vectors.
+EMBEDDING_MODEL_NAME = os.environ.get("EMBEDDING_MODEL_NAME", "BAAI/bge-small-en-v1.5")
+BATCH_SIZE = int(os.environ.get("BATCH_SIZE", "100"))
+EMBEDDING_BATCH_SIZE = int(os.environ.get("EMBEDDING_BATCH_SIZE", "64"))
+
 METADATA_COLLECTION = "mcp_indexer_metadata"
 INCREMENTAL_DAYS = int(os.environ.get("INCREMENTAL_DAYS", "7"))
 FORCE_REINDEX = os.environ.get("FORCE_REINDEX", "").lower() in ("1", "true", "yes")