feat: Vorbereiter-Agent (Preparer) für Sessions

- Neues optionales Session-Feld 'preparerAgentId' (DB + Model + API)
- PreparerContext im ChatContext für Agent-Konfiguration
- RunPreparer() in AnthropicService: führt Preparer vor dem Worker aus
  - [FORWARD] → aufbereiteter Prompt wird an Session-Agent weitergegeben
  - Ohne [FORWARD] → Rückfrage an User, Flow stoppt (preparer_query)
- Preparer-System-Prompt: erklärt Rolle, [FORWARD]-Mechanismus, Vorarbeit
- Preparer kann eigene Tools + Fähigkeiten aktiv nutzen
- ChatEndpoint: Preparer-Phase vor StreamChat integriert
- Frontend: SSE-Events (preparer_start/text/tool_use/done)
- Preparer-Bubble (Info-Farbe) mit Status Weitergeleitet/Rückfrage
- [Preparer: Name] → hübscher Header, [FORWARD] ausgeblendet
- [Vorbereitet von Name] → dezenter Header in User-Bubble
- Agent-Picker im Session-Form (vor Review-Agent)

Services/DatabaseService.cs

@@ -180,6 +180,7 @@ public class DatabaseService
         TryAddColumn(conn, "sessions", "finalizer_agent_id", "TEXT");
         TryAddColumn(conn, "agents", "allow_user_choice", "INTEGER NOT NULL DEFAULT 1");
         TryAddColumn(conn, "chains", "plan_mode", "INTEGER NOT NULL DEFAULT 0");
+        TryAddColumn(conn, "sessions", "preparer_agent_id", "TEXT");
 
         conn.Execute(@"
             CREATE TABLE IF NOT EXISTS agent_skills (
@@ -377,7 +378,7 @@ public class DatabaseService
 
     // ── Sessions ──
 
-    public async Task<SessionRecord> CreateSession(string userId, string name = null, string description = null, string image = null, string agentId = null, string workspace = null, string context = null, string customCommands = null, double avatarScale = 1.0, double avatarOffsetX = 0, double avatarOffsetY = 0, string reviewAgentId = null, int maxReviewIterations = 3, string finalizerAgentId = null)
+    public async Task<SessionRecord> CreateSession(string userId, string name = null, string description = null, string image = null, string agentId = null, string workspace = null, string context = null, string customCommands = null, double avatarScale = 1.0, double avatarOffsetX = 0, double avatarOffsetY = 0, string reviewAgentId = null, int maxReviewIterations = 3, string finalizerAgentId = null, string preparerAgentId = null)
     {
         using var conn = CreateConnection();
         var now = DateTime.UtcNow.ToString("o");
@@ -398,12 +399,13 @@ public class DatabaseService
             ReviewAgentId = reviewAgentId,
             MaxReviewIterations = maxReviewIterations,
             FinalizerAgentId = finalizerAgentId,
+            PreparerAgentId = preparerAgentId,
             CreatedAt = now,
             UpdatedAt = now
         };
         await conn.ExecuteAsync(@"
-            INSERT INTO sessions (session_id, user_id, name, description, image, agent_id, workspace, context, custom_commands, avatar_scale, avatar_offset_x, avatar_offset_y, review_agent_id, max_review_iterations, finalizer_agent_id, created_at, updated_at)
-            VALUES (@SessionId, @UserId, @Name, @Description, @Image, @AgentId, @Workspace, @Context, @CustomCommands, @AvatarScale, @AvatarOffsetX, @AvatarOffsetY, @ReviewAgentId, @MaxReviewIterations, @FinalizerAgentId, @CreatedAt, @UpdatedAt)
+            INSERT INTO sessions (session_id, user_id, name, description, image, agent_id, workspace, context, custom_commands, avatar_scale, avatar_offset_x, avatar_offset_y, review_agent_id, max_review_iterations, finalizer_agent_id, preparer_agent_id, created_at, updated_at)
+            VALUES (@SessionId, @UserId, @Name, @Description, @Image, @AgentId, @Workspace, @Context, @CustomCommands, @AvatarScale, @AvatarOffsetX, @AvatarOffsetY, @ReviewAgentId, @MaxReviewIterations, @FinalizerAgentId, @PreparerAgentId, @CreatedAt, @UpdatedAt)
         ", session);
         return session;
     }
@@ -452,6 +454,7 @@ public class DatabaseService
                 s.ReviewAgentId,
                 s.MaxReviewIterations,
                 s.FinalizerAgentId,
+                s.PreparerAgentId,
                 s.IsDeleted,
                 s.CreatedAt,
                 s.UpdatedAt,
@@ -507,6 +510,7 @@ public class DatabaseService
         if (body.TryGetProperty("reviewAgentId", out var ra)) { sets.Add("review_agent_id = @reviewAgentId"); p.Add("reviewAgentId", ra.ValueKind == System.Text.Json.JsonValueKind.Null ? null : ra.GetString()); }
         if (body.TryGetProperty("maxReviewIterations", out var mri)) { sets.Add("max_review_iterations = @maxReviewIterations"); p.Add("maxReviewIterations", mri.GetInt32()); }
         if (body.TryGetProperty("finalizerAgentId", out var fa)) { sets.Add("finalizer_agent_id = @finalizerAgentId"); p.Add("finalizerAgentId", fa.ValueKind == System.Text.Json.JsonValueKind.Null ? null : fa.GetString()); }
+        if (body.TryGetProperty("preparerAgentId", out var pa)) { sets.Add("preparer_agent_id = @preparerAgentId"); p.Add("preparerAgentId", pa.ValueKind == System.Text.Json.JsonValueKind.Null ? null : pa.GetString()); }
 
         await conn.ExecuteAsync($"UPDATE sessions SET {string.Join(", ", sets)} WHERE session_id = @sessionId", p);
     }