test(todos): schémas Pydantic + 9 tests d'intégration todos (en échec)

Ajoute les schémas Pydantic TodoCreate/TodoUpdate/PostponeRequest/TodoResponse,
la fixture db_session dans conftest, et 9 tests d'intégration contre PostgreSQL
réel — tous en échec car les endpoints /api/todos/ n'existent pas encore.

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

backend/app/schemas/todos.py

@@ -0,0 +1,50 @@
+import uuid
+from datetime import datetime
+from pydantic import BaseModel, ConfigDict
+
+
+class TodoCreate(BaseModel):
+    title: str
+    body: str | None = None
+    url: str | None = None
+    domain: str | None = None
+    category: str | None = None
+    tags: list[str] = []
+    status: str = "pending"
+    priority: str = "medium"
+    due_date: datetime | None = None
+
+
+class TodoUpdate(BaseModel):
+    title: str | None = None
+    body: str | None = None
+    url: str | None = None
+    domain: str | None = None
+    category: str | None = None
+    tags: list[str] | None = None
+    status: str | None = None
+    priority: str | None = None
+    due_date: datetime | None = None
+
+
+class PostponeRequest(BaseModel):
+    days: int  # 1 ou 7
+
+
+class TodoResponse(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: uuid.UUID
+    title: str
+    body: str | None
+    url: str | None
+    domain: str | None
+    category: str | None
+    tags: list[str]
+    status: str
+    priority: str
+    due_date: datetime | None
+    postponed_count: int
+    created_at: datetime
+    updated_at: datetime | None
+    owner_id: uuid.UUID | None