feat(billing): extract Billing Service (Step 4)

- stripe_service: checkout sessions, webhook handling, subscription CRUD
- tier_manager: feature matrix (4 tiers), quota enforcement, rate limits
- routes: checkout, webhook (no auth), subscription, tier query, features
- Traefik header auth (X-User-Id) replaces get_current_user dependency
- /tier/{user_id} endpoint for internal service-to-service lookups
- /features and /features/{tier} for feature matrix queries
- Dockerfile: single worker, 30s timeout (lightweight service)

services/billing/app/routes.py

@@ -0,0 +1,134 @@
+"""Billing routes: Stripe checkout, webhook, subscription, tier query.
+
+Adapted for the Billing microservice:
+  - Authenticated routes use Traefik-injected headers (X-User-Id, X-User-Tier)
+  - Webhook route has NO auth (Stripe signature verification only)
+  - Added /tier/{user_id} for internal service-to-service tier lookups
+  - Added /features/{tier} for feature matrix queries
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from fastapi import APIRouter, Header, HTTPException, Request, status
+from pydantic import BaseModel
+
+from shared.db import async_session
+from shared.schemas import BillingTier
+
+from app.stripe_service import stripe_service
+from app.tier_manager import tier_manager, FEATURES, RATE_LIMITS
+
+router = APIRouter(prefix="/billing", tags=["billing"])
+
+
+# ── Request bodies ─────────────────────────────────────────────────────
+
+class _CheckoutRequest(BaseModel):
+    tier: BillingTier
+
+
+# ── Checkout ───────────────────────────────────────────────────────────
+
+@router.post("/checkout")
+async def create_checkout(
+    body: _CheckoutRequest,
+    x_user_id: str = Header(..., alias="X-User-Id"),
+) -> dict[str, str]:
+    """Create a Stripe checkout session for a tier upgrade."""
+    url = stripe_service.create_checkout_session(x_user_id, body.tier)
+    return {"checkout_url": url}
+
+
+# ── Webhook (NO auth — Stripe signature only) ─────────────────────────
+
+@router.post("/webhook")
+async def stripe_webhook(
+    request: Request,
+    stripe_signature: str = Header(default="", alias="Stripe-Signature"),
+) -> dict[str, bool]:
+    """Handle Stripe webhook events.
+
+    This endpoint is exposed without ForwardAuth in Traefik config
+    so Stripe can reach it directly.
+    """
+    payload = await request.body()
+    async with async_session() as db:
+        await stripe_service.handle_webhook(payload, stripe_signature, db)
+    return {"ok": True}
+
+
+# ── Subscription CRUD ─────────────────────────────────────────────────
+
+@router.get("/subscription")
+async def get_subscription(
+    x_user_id: str = Header(..., alias="X-User-Id"),
+    x_user_tier: str = Header("free", alias="X-User-Tier"),
+) -> dict[str, Any]:
+    """Return the current subscription info for the authenticated user."""
+    async with async_session() as db:
+        sub = await stripe_service.get_subscription(x_user_id, db)
+    if sub is None:
+        return {
+            "tier": x_user_tier,
+            "status": "free",
+            "stripe_subscription_id": None,
+            "current_period_end": None,
+        }
+    return sub
+
+
+@router.delete("/subscription")
+async def cancel_subscription(
+    x_user_id: str = Header(..., alias="X-User-Id"),
+) -> dict[str, bool]:
+    """Cancel the active subscription."""
+    async with async_session() as db:
+        await stripe_service.cancel_subscription(x_user_id, db)
+    return {"ok": True}
+
+
+# ── Tier query (internal, service-to-service) ─────────────────────────
+
+@router.get("/tier/{user_id}")
+async def get_user_tier(user_id: str) -> dict[str, str]:
+    """Return the billing tier for a given user_id.
+
+    Used by other services for tier lookups. Protected by Traefik
+    ForwardAuth — only internal services should call this.
+    """
+    async with async_session() as db:
+        tier = await tier_manager.get_tier(user_id, db)
+    return {"user_id": user_id, "tier": tier}
+
+
+# ── Feature matrix (public, cacheable) ────────────────────────────────
+
+@router.get("/features/{tier}")
+async def get_tier_features(tier: str) -> dict[str, Any]:
+    """Return the feature matrix for a tier."""
+    if tier not in FEATURES:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail=f"Unknown tier: {tier}",
+        )
+    return {
+        "tier": tier,
+        "features": FEATURES[tier],
+        "rate_limit_rpm": RATE_LIMITS.get(tier, RATE_LIMITS["free"]),
+    }
+
+
+@router.get("/features")
+async def get_all_features() -> dict[str, Any]:
+    """Return the full feature matrix for all tiers."""
+    return {
+        "tiers": {
+            tier: {
+                "features": features,
+                "rate_limit_rpm": RATE_LIMITS.get(tier, RATE_LIMITS["free"]),
+            }
+            for tier, features in FEATURES.items()
+        },
+    }