feat(api): report login methods and add GET /capabilities

api/openapi/components/schemas/Capabilities.yaml

@@ -0,0 +1,10 @@
+type: object
+description: |
+  Capability modules this server implements, keyed by module. Keys are optional; a missing key means the
+  module is not implemented. New modules are added as new optional keys. These are server facts, not what
+  the calling token may use.
+properties:
+  core:
+    $ref: ./CoreCapability.yaml
+  password:
+    $ref: ./PasswordCapability.yaml

api/openapi/components/schemas/CoreCapability.yaml

@@ -0,0 +1,5 @@
+type: object
+description: The mandatory core module.
+required: [version]
+properties:
+  version: {type: integer, description: Module version. Bumped only on semantic change.}

api/openapi/components/schemas/LoginMethods.yaml

@@ -0,0 +1,8 @@
+type: object
+description: |
+  Login methods this server accepts, keyed by method. A missing key means the method is not offered.
+  Keys are optional on purpose: discovery is read by clients of any version against servers of any
+  version, so new methods are added as new optional keys. Clients ignore keys they do not know.
+properties:
+  password:
+    $ref: ./PasswordLoginMethod.yaml

api/openapi/components/schemas/PasswordCapability.yaml

@@ -0,0 +1,5 @@
+type: object
+description: The password login module (login, first-admin setup, password change).
+required: [version]
+properties:
+  version: {type: integer, description: Module version. Bumped only on semantic change.}

api/openapi/components/schemas/PasswordLoginMethod.yaml

@@ -0,0 +1,2 @@
+type: object
+description: Username and password login (`POST /auth/login`). No settings yet.

api/openapi/components/schemas/ServerInfo.yaml

@@ -15,8 +15,4 @@ properties:
     type: boolean
     description: True until the first admin user has been created.
   loginMethods:
-    type: array
-    description: Login methods this server accepts. New methods may be added; clients ignore values they do not recognise.
-    items:
-      type: string
-      enum: [password]
+    $ref: ./LoginMethods.yaml