feat: admin endpoints to reset username sync flags

Add POST /v1/admin/users/{pubkey}/reset-username and
POST /v1/admin/users/reset-usernames to clear manual_username
and last_synced_at so nostr profile sync re-evaluates users.
Includes OpenAPI docs, audit actions, and tests.

internal/http/docs/openapi.yaml

@@ -290,6 +290,48 @@ paths:
         '200': { description: Deleted }
         '401': { description: Unauthorized }
         '404': { description: Not found }
+  /v1/admin/users/reset-usernames:
+    post:
+      tags: [Admin]
+      summary: Reset all usernames (admin)
+      description: |
+        Clears `manual_username` and `last_synced_at` for every active user so
+        the nostr profile sync worker re-evaluates each one on its next cycle.
+        The stored username is left untouched until the worker overwrites it
+        with fresh kind:0 metadata. Idempotent.
+      security: [{ AdminAPIKey: [] }]
+      responses:
+        '200':
+          description: Reset count
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  reset: { type: integer, description: "Number of users whose sync flags were cleared" }
+        '401': { description: Unauthorized }
+  /v1/admin/users/{pubkey}/reset-username:
+    post:
+      tags: [Admin]
+      summary: Reset username for a user (admin)
+      description: |
+        Clears `manual_username` and `last_synced_at` for the given user so the
+        nostr profile sync worker re-evaluates their kind:0 metadata on its next
+        cycle. The stored username is left untouched until the worker overwrites
+        it. Idempotent.
+      security: [{ AdminAPIKey: [] }]
+      parameters:
+        - in: path
+          name: pubkey
+          required: true
+          schema: { type: string }
+      responses:
+        '200':
+          description: Reset
+          content: { application/json: { schema: { $ref: '#/components/schemas/User' } } }
+        '400': { description: Validation error, content: { application/json: { schema: { $ref: '#/components/schemas/Error' } } } }
+        '401': { description: Unauthorized }
+        '404': { description: Not found }
   /v1/admin/users/{pubkey}/extend:
     post:
       tags: [Admin]