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/handlers/presence.go

@@ -81,3 +81,38 @@ func (h *PresenceHandler) SearchUsers(c *gin.Context) {
 	}
 	c.JSON(http.StatusOK, gin.H{"data": results})
 }
+
+// ResolveUsers returns identity records for a batch of user IDs.
+// GET /api/v1/users/resolve?ids=uuid1,uuid2,...
+func (h *PresenceHandler) ResolveUsers(c *gin.Context) {
+	raw := strings.TrimSpace(c.Query("ids"))
+	if raw == "" {
+		c.JSON(http.StatusOK, gin.H{"data": map[string]interface{}{}})
+		return
+	}
+
+	ids := make([]string, 0)
+	for _, id := range strings.Split(raw, ",") {
+		id = strings.TrimSpace(id)
+		if id != "" {
+			ids = append(ids, id)
+		}
+	}
+	if len(ids) == 0 {
+		c.JSON(http.StatusOK, gin.H{"data": map[string]interface{}{}})
+		return
+	}
+
+	results, err := h.stores.Users.ResolveByIDs(c.Request.Context(), ids)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "resolve failed"})
+		return
+	}
+
+	// Build map keyed by user ID for O(1) client-side lookups
+	m := make(map[string]interface{}, len(results))
+	for _, u := range results {
+		m[u.ID] = u
+	}
+	c.JSON(http.StatusOK, gin.H{"data": m})
+}