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

Made-with: Cursor

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

@@ -0,0 +1,33 @@
+post:
+  tags: [Auth]
+  summary: Sign in with npub only (limited)
+  requestBody:
+    content:
+      application/json:
+        schema:
+          type: object
+          required: [npub]
+          properties:
+            npub:
+              type: string
+  responses:
+    "200":
+      description: Token and pubkey
+      content:
+        application/json:
+          schema:
+            type: object
+            properties:
+              token:
+                type: string
+              pubkey:
+                type: string
+              method:
+                type: string
+                example: npub
+    "400":
+      description: Invalid npub
+      content:
+        application/json:
+          schema:
+            $ref: "../components/schemas/common.yaml#/ApiError"