Initial commit: Backend API Phase 1a complete

- FastAPI backend with JWT auth
- PostgreSQL database schema
- Trees and Sessions CRUD APIs
- Export functionality (Markdown, Text, HTML)
- Docker setup for local development
- Alembic migrations

backend/app/schemas/token.py

@@ -0,0 +1,14 @@
+from typing import Optional
+from pydantic import BaseModel
+
+
+class Token(BaseModel):
+    access_token: str
+    refresh_token: str
+    token_type: str = "bearer"
+
+
+class TokenPayload(BaseModel):
+    sub: Optional[str] = None
+    exp: Optional[int] = None
+    type: Optional[str] = None