refactor: factorise la création d'engine et normalise solde_montant

- connection: création de l'engine centralisée dans _build_engine (source
  unique de config) ; init_db réutilise reset_connection + get_engine au lieu
  de dupliquer create_engine
- service: _normalize_amount garantit qu'un montant non numérique issu de
  l'extraction (string, dict…) n'entre jamais en base dans une colonne Float ;
  appliqué à solde_montant
- tests: couverture de _normalize_amount et du solde sous forme de string

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/plesna_gerance/database/connection.py

@@ -36,23 +36,27 @@ def get_db_path() -> Path:
     return DEFAULT_DB_PATH
 
 
+def _build_engine(db_path: Path):
+    """Create a SQLAlchemy engine for the given SQLite path.
+
+    Single source of truth for engine configuration.
+    """
+    # Create parent directory if needed
+    db_path.parent.mkdir(parents=True, exist_ok=True)
+
+    return create_engine(
+        f"sqlite:///{db_path}",
+        echo=False,  # Set to True for SQL debugging
+        connect_args={"check_same_thread": False},  # Required for FastAPI
+    )
+
+
 def get_engine(db_path: Path | None = None):
     """Get or create SQLAlchemy engine (singleton pattern)."""
     global _engine
 
     if _engine is None:
-        if db_path is None:
-            db_path = get_db_path()
-
-        # Create parent directory if needed
-        db_path.parent.mkdir(parents=True, exist_ok=True)
-
-        # Create engine with SQLite
-        _engine = create_engine(
-            f"sqlite:///{db_path}",
-            echo=False,  # Set to True for SQL debugging
-            connect_args={"check_same_thread": False},  # Required for FastAPI
-        )
+        _engine = _build_engine(db_path or get_db_path())
 
     return _engine
 
@@ -87,25 +91,13 @@ def init_db(db_path: Path | None = None) -> Path:
     if db_path is None:
         db_path = get_db_path()
 
-    # Reset globals to use new path
-    global _engine, _SessionLocal
-    _engine = None
-    _SessionLocal = None
-
-    # Create parent directory
-    db_path.parent.mkdir(parents=True, exist_ok=True)
-
-    # Create engine and tables
-    engine = create_engine(
-        f"sqlite:///{db_path}", echo=False, connect_args={"check_same_thread": False}
-    )
+    # Reset globals so the engine/session factory rebuild against db_path
+    reset_connection()
 
+    # Build the engine (reuses the shared configuration) and create tables
+    engine = get_engine(db_path)
     Base.metadata.create_all(bind=engine)
 
-    # Update globals
-    _engine = engine
-    _SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
-
     # Seed predefined tags if the table is empty
     _seed_tags_if_empty(engine)