fix: replace sql.NullTime with string scan for MAX() aggregate queries

Go 1.26 changed NullTime.Scan to delegate to convertAssign,
which has no string->time.Time conversion path. The
modernc.org/sqlite driver returns raw strings for aggregate
expressions like MAX(last_used_at), causing silent scan failures
that made all clients/providers show 'Never' for last used.

Fixes by scanning into a string and parsing with time.Parse.

internal/server/clients.go

@@ -1,7 +1,6 @@
 package server
 
 import (
-	"database/sql"
 	"net/http"
 	"time"
 
@@ -47,14 +46,15 @@ func (s *Server) handleGetClients(c *gin.Context) {
 			desc = *cl.Description
 		}
 
-		var lastUsedTime sql.NullTime
+		var lastUsedStr string
-		_ = s.database.Get(&lastUsedTime, "SELECT MAX(last_used_at) FROM client_tokens WHERE client_id = ?", cl.ClientID)
+		_ = s.database.Get(&lastUsedStr, "SELECT MAX(last_used_at) FROM client_tokens WHERE client_id = ?", cl.ClientID)
 
 		var lastUsed *time.Time
-		if lastUsedTime.Valid && !lastUsedTime.Time.IsZero() {
+		if lastUsedStr != "" {
-			t := lastUsedTime.Time
+			if t, err := time.Parse("2006-01-02 15:04:05", lastUsedStr); err == nil {
 				lastUsed = &t
 			}
+		}
 
 		uiClients[i] = UIClient{
 			ID:                 cl.ClientID,

internal/server/providers_admin.go

@@ -1,7 +1,6 @@
 package server
 
 import (
-	"database/sql"
 	"fmt"
 	"net/http"
 	"strings"
@@ -87,11 +86,13 @@ func (s *Server) handleGetProviders(c *gin.Context) {
 		}
 
 		// Get last used for this provider
-		var lastUsedTime sql.NullTime
+		var lastUsedStr string
-		_ = s.database.Get(&lastUsedTime, "SELECT MAX(timestamp) FROM llm_requests WHERE provider = ?", id)
+		_ = s.database.Get(&lastUsedStr, "SELECT MAX(timestamp) FROM llm_requests WHERE provider = ?", id)
 		var lastUsed interface{}
-		if lastUsedTime.Valid && !lastUsedTime.Time.IsZero() {
+		if lastUsedStr != "" {
-			lastUsed = lastUsedTime.Time
+			if t, err := time.Parse("2006-01-02 15:04:05", lastUsedStr); err == nil {
+				lastUsed = t
+			}
 		}
 
 		// Get models for this provider from registry