feat: add database schema with WAL mode and FTS5 triggers

SQLite database layer with thread-local connections, WAL journal mode,
foreign key enforcement, and FTS5 full-text search on utterances via
content-table triggers. TDD: 5 tests written first, all passing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/conftest.py

@@ -0,0 +1,39 @@
+import pytest
+import os
+from app.shared.config import Settings
+from app.shared.database import init_schema, get_db
+
+
+@pytest.fixture
+def settings(tmp_path):
+    db_file = str(tmp_path / "test.db")
+    clips_dir = str(tmp_path / "clips")
+    os.makedirs(clips_dir, exist_ok=True)
+    s = Settings(
+        db_path=db_file,
+        clips_dir=clips_dir,
+        speaches_url="http://localhost:9999",
+        ollama_url="http://localhost:9999",
+        livekit_api_key="test",
+        livekit_api_secret="test",
+    )
+    init_schema(s)
+    return s
+
+
+@pytest.fixture
+def db(settings):
+    return get_db(settings)
+
+
+# Add to conftest.py
+from app.shared import database as _db_module
+
+@pytest.fixture(autouse=True)
+def reset_db_connection():
+    """Reset thread-local DB connection between tests."""
+    _db_module._local.conn = None
+    yield
+    if hasattr(_db_module._local, 'conn') and _db_module._local.conn:
+        _db_module._local.conn.close()
+        _db_module._local.conn = None