refactor: per-operation DB sessions and ServiceError in SessionService

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_integration.py

@@ -3,6 +3,7 @@ from unittest.mock import AsyncMock, MagicMock
 
 from sqlalchemy import create_engine
 from sqlalchemy.orm import sessionmaker
+from sqlalchemy.pool import StaticPool
 
 from hqt.db.models import Base, Harness
 from hqt.projects.service import ProjectService
@@ -13,9 +14,13 @@ from hqt.harnesses.configurators.claude import ClaudeConfigurator
 
 @pytest.fixture
 def setup():
-    engine = create_engine("sqlite:///:memory:")
+    engine = create_engine(
+        "sqlite://",
+        connect_args={"check_same_thread": False},
+        poolclass=StaticPool,
+    )
     Base.metadata.create_all(engine)
-    factory = sessionmaker(bind=engine)
+    factory = sessionmaker(bind=engine, expire_on_commit=False)
     db = factory()
     db.add(Harness(name="claude-code", display_name="Claude Code"))
     db.commit()
@@ -32,8 +37,8 @@ def setup():
     configurator = ClaudeConfigurator()
     harnesses = {"claude-code": configurator}
 
-    proj_svc = ProjectService(db)
-    sess_svc = SessionService(db=db, tmux=tmux, harnesses=harnesses)
+    proj_svc = ProjectService(factory)
+    sess_svc = SessionService(factory=factory, tmux=tmux, harnesses=harnesses)
     return proj_svc, sess_svc, tmux