feat(slice-1): FastAPI factory + /healthz; self-migrate at startup (INV-1/§6.4)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/main.py

@@ -1,5 +1,51 @@
 """ecomm backend — FastAPI app factory + the REST BFF.
 
-Stub in SLICE-1 Task 1 so the top layer of the import-linter contract exists from the
-start; the FastAPI factory and /healthz are added in Task 5.
+SLICE-1 mounts only /healthz; the auth and storefront endpoints (§6.4) grow here in
+later slices. create_app() opens the connection pool and self-migrates the database
+at startup (INV-1, INV-7): the app boots against empty persistence and applies its
+own schema — there is no seed step.
 """
+from __future__ import annotations
+
+from contextlib import asynccontextmanager
+
+import psycopg
+from fastapi import Depends, FastAPI, Response
+
+from app.platform import config, db
+from app.platform.deps import get_conn
+
+
+def create_app(database_url: str | None = None) -> FastAPI:
+    dsn = database_url or config.database_url()
+
+    @asynccontextmanager
+    async def lifespan(app: FastAPI):
+        app.state.pool = db.open_pool(dsn)
+        with app.state.pool.connection() as conn:
+            db.migrate(conn)  # self-migrate at startup (INV-1, INV-7)
+        try:
+            yield
+        finally:
+            app.state.pool.close()
+
+    app = FastAPI(title="ecomm", version="0.1", lifespan=lifespan)
+
+    @app.get("/healthz")
+    def healthz(response: Response, conn: psycopg.Connection = Depends(get_conn)):
+        """Liveness + readiness: process up, DB reachable, migrations current (§6.4)."""
+        try:
+            conn.execute("SELECT 1")
+            pending = db.pending_migrations(conn)
+        except Exception:
+            response.status_code = 503
+            return {"status": "unavailable", "reason": "database_unreachable"}
+        if pending:
+            response.status_code = 503
+            return {"status": "unavailable", "reason": "migrations_pending"}
+        return {"status": "ok"}
+
+    return app
+
+
+app = create_app()