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/test_database.py

@@ -0,0 +1,41 @@
+def test_schema_creates_rooms_table(db):
+    rows = db.execute("SELECT name FROM sqlite_master WHERE type='table' AND name='rooms'").fetchall()
+    assert len(rows) == 1
+
+
+def test_schema_creates_utterances_fts(db):
+    rows = db.execute("SELECT name FROM sqlite_master WHERE type='table' AND name='utterances_fts'").fetchall()
+    assert len(rows) == 1
+
+
+def test_insert_and_fetch_room(db):
+    db.execute("INSERT INTO rooms (name) VALUES (?)", ("Kitchen",))
+    db.commit()
+    row = db.execute("SELECT name FROM rooms WHERE name='Kitchen'").fetchone()
+    assert row["name"] == "Kitchen"
+
+
+def test_insert_speaker_and_embedding(db):
+    db.execute("INSERT INTO speakers (name) VALUES (?)", ("Jeremy",))
+    db.commit()
+    speaker = db.execute("SELECT id FROM speakers WHERE name='Jeremy'").fetchone()
+    db.execute(
+        "INSERT INTO voice_embeddings (speaker_id, embedding) VALUES (?, ?)",
+        (speaker["id"], b"\x00\x01\x02\x03"),
+    )
+    db.commit()
+    emb = db.execute("SELECT speaker_id FROM voice_embeddings").fetchone()
+    assert emb["speaker_id"] == speaker["id"]
+
+
+def test_fts_trigger_on_utterance_insert(db):
+    db.execute("INSERT INTO rooms (name) VALUES (?)", ("Living Room",))
+    db.commit()
+    room = db.execute("SELECT id FROM rooms WHERE name='Living Room'").fetchone()
+    db.execute(
+        "INSERT INTO utterances (room_id, transcript, match_status, start_time, end_time) VALUES (?,?,?,?,?)",
+        (room["id"], "hello world test", "unreviewed", "2026-01-01 10:00:00", "2026-01-01 10:00:03"),
+    )
+    db.commit()
+    results = db.execute("SELECT rowid FROM utterances_fts WHERE transcript MATCH 'hello'").fetchall()
+    assert len(results) == 1