Feat v0.6.15 user display audit

Add batch user resolve endpoint, sw.users SDK module, and migrate all
surfaces to show display_name instead of UUIDs. Canonical fallback chain:
display_name → username → "Unknown".

- GET /api/v1/users/resolve?ids=... (max 100, map response)
- sw.users.resolve(), resolveMany(), displayName() with 60s cache
- Chat participants resolved from users table, not snapshot
- Admin users/teams/groups/team-admin show display_name
- Chat-core participants.display_name column deprecated
- 5 new handler tests, OpenAPI spec updated

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/static/openapi.yaml

@@ -1355,6 +1355,42 @@ paths:
                     items:
                       type: object
 
+  /api/v1/users/resolve:
+    get:
+      summary: Resolve user identities by ID
+      operationId: resolveUsers
+      tags: [Presence]
+      parameters:
+        - name: ids
+          in: query
+          required: true
+          description: Comma-separated user IDs (max 100)
+          schema:
+            type: string
+      responses:
+        '200':
+          description: Map of user ID to identity record
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  data:
+                    type: object
+                    additionalProperties:
+                      type: object
+                      properties:
+                        id:
+                          type: string
+                        username:
+                          type: string
+                        display_name:
+                          type: string
+                        handle:
+                          type: string
+                        avatar_url:
+                          type: string
+
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   # Notifications
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