Add Swagger docs at /docs and /openapi.json with split OpenAPI spec

Made-with: Cursor

backend/openapi/path-items/auth-login.yaml

@@ -0,0 +1,20 @@
+post:
+  tags: [Auth]
+  summary: Sign in with NIP-98 (returns JWT)
+  security:
+    - NIP98: []
+  responses:
+    "200":
+      description: Token and pubkey
+      content:
+        application/json:
+          schema:
+            type: object
+            properties:
+              token:
+                type: string
+              pubkey:
+                type: string
+              method:
+                type: string
+                example: nip98