test: add indexer unit tests + integration marker; make indexer importable without env

- pytest.ini: 'integration' marker, deselected by default; the integration
  suite requires a live Qdrant and will only run in the nightly CI job.
- src/indexer.py: move env validation from import time into _validate_config()
  called by main() — the module can now be imported without MAILDIR_PATH /
  QDRANT_URL / COLLECTION_NAME set, enabling unit tests.
- tests/test_indexer.py: 27 new unit tests, no network and no model:
  decode_mime_words (encoded-words utf-8/iso-8859-1, mixed), extract_text_from_html
  (tags, scripts), normalize_email_address, parse_email_message with real
  EmailMessage objects (plain bodies, attachments detected and excluded from
  body, HTML alternative, unknown charset fallback via raw bytes message,
  encoded attachment filename), get_recent_keys against a real tmp_path
  Maildir (fresh mtime selected, old mtime excluded, colon-suffix keys).
  mailbox.Maildir(create=True) does not create cur/new/tmp on Python 3.11 —
  the test helper pre-creates them.

pytest.ini

@@ -1,3 +1,8 @@
 [pytest]
 testpaths = tests
 pythonpath = src
+; Integration tests (tests/test_integration.py) require a live Qdrant on
+; localhost:6333 — they only run in the nightly CI job (or manually with -m integration).
+markers =
+    integration: requires a live Qdrant instance (deselected by default)
+addopts = -m "not integration" -p no:cacheprovider