feat: API Key pro Agent statt pro User

- Neue Tabelle agent_api_keys (user_id, agent_id, key_id)
- Migration: bestehende selected_api_key_id auf alle Agents verteilt
- Backend: GetAgentApiKeyRecord(userId, agentId) in Chat, Telegram, Cronjobs
- Endpoints: select/selected mit ?agentId Query-Parameter
- Frontend: currentAgentId aus Session, alle Key-Calls agent-aware
- DeleteApiKey bereinigt agent_api_keys statt users.selected_api_key_id

Services/DatabaseService.cs

@@ -357,9 +357,23 @@ public partial class DatabaseService
         TryAddColumn(conn, "api_keys", "source_url", "TEXT");
         TryAddColumn(conn, "api_keys", "is_public", "INTEGER NOT NULL DEFAULT 0");
 
-        // users: ausgewählter API-Key
+        // users: ausgewählter API-Key (legacy, wird durch agent_api_keys ersetzt)
         TryAddColumn(conn, "users", "selected_api_key_id", "TEXT");
 
+        conn.Execute(@"
+            CREATE TABLE IF NOT EXISTS agent_api_keys (
+                id        INTEGER PRIMARY KEY AUTOINCREMENT,
+                user_id   TEXT NOT NULL,
+                agent_id  TEXT NOT NULL,
+                key_id    TEXT NOT NULL,
+                UNIQUE(user_id, agent_id)
+            );
+            CREATE INDEX IF NOT EXISTS idx_agent_api_keys_user ON agent_api_keys(user_id);
+        ");
+
+        // Migration: selected_api_key_id → agent_api_keys (einmalig)
+        MigrateSelectedApiKeysToAgentKeys(conn);
+
         // Migration: Bestehende PNG/JPEG-Avatare/Images zu WebP konvertieren
         MigrateImagesToWebP(conn);
 
@@ -367,6 +381,43 @@ public partial class DatabaseService
         MigrateImageThumbnails(conn);
     }
 
+    private void MigrateSelectedApiKeysToAgentKeys(SqliteConnection conn)
+    {
+        // Nur migrieren wenn agent_api_keys leer ist und users mit selected_api_key_id existieren
+        var count = conn.ExecuteScalar<int>("SELECT COUNT(*) FROM agent_api_keys");
+        if (count > 0) return;
+
+        var usersWithKeys = conn.Query<(string UserId, string SelectedApiKeyId)>(
+            "SELECT user_id, selected_api_key_id FROM users WHERE selected_api_key_id IS NOT NULL AND selected_api_key_id != ''")
+            .ToList();
+
+        foreach (var (userId, keyId) in usersWithKeys)
+        {
+            // Alle Agents des Users + öffentliche Agents
+            var agentIds = conn.Query<string>(
+                "SELECT agent_id FROM agents WHERE user_id = @userId OR is_public = 1",
+                new { userId }).ToList();
+
+            // Alle Sessions des Users mit Agent → deren Agents auch berücksichtigen
+            var sessionAgentIds = conn.Query<string>(
+                "SELECT DISTINCT agent_id FROM sessions WHERE user_id = @userId AND agent_id IS NOT NULL AND agent_id != ''",
+                new { userId }).ToList();
+
+            var allAgentIds = agentIds.Union(sessionAgentIds).Distinct().ToList();
+
+            foreach (var agentId in allAgentIds)
+            {
+                try
+                {
+                    conn.Execute(
+                        "INSERT OR IGNORE INTO agent_api_keys (user_id, agent_id, key_id) VALUES (@userId, @agentId, @keyId)",
+                        new { userId, agentId, keyId });
+                }
+                catch { /* UNIQUE constraint – already exists */ }
+            }
+        }
+    }
+
     private void MigrateImagesToWebP(SqliteConnection conn)
     {
         const int webPQuality = 80;