Add authentication and user profile endpoint

Argon2 password hashing, JWT session issuing/verification
(auth/security.py), register/login routes, the bearer-token
get_current_user dependency, and GET /users/me for address + balance.

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

app/api/routes/users.py

@@ -0,0 +1,18 @@
+from fastapi import APIRouter, Depends
+from pydantic import BaseModel
+
+from app.auth.dependencies import get_current_user
+from app.db.models import User
+
+router = APIRouter(prefix="/users", tags=["users"])
+
+
+class MeResponse(BaseModel):
+    username: str
+    address: str
+    balance_sats: int
+
+
+@router.get("/me", response_model=MeResponse)
+async def me(user: User = Depends(get_current_user)) -> MeResponse:
+    return MeResponse(username=user.username, address=user.address, balance_sats=user.cached_balance_sats)