test: add pytest suite - 97 tests, search + indexer + auth

Create comprehensive test suite with 97 passing tests:
- tests/conftest.py: fixtures (TestClient, temp vault dirs, index setup)
- tests/test_search.py (27 tests): tokenizer, snippets, highlight,
  tag filter, search API, advanced search, suggest, tags API
- tests/test_indexer.py (32 tests): frontmatter parsing, inline tags,
  title extraction, scan_vault, find_file_in_index, backlinks
- tests/test_auth.py (38 tests): password hashing, JWT create/decode,
  token revocation, user CRUD, login lockout, rate limiting, middleware

Also fix: lazy WeasyPrint import (graceful fallback when GTK missing),
add data/ to .gitignore (runtime files from test runs).

.gitignore

@@ -11,3 +11,4 @@ venv/
 dist/
 build/
 config.json
+data/