fix(plugins): ai_assistant Migrationen von gedroppten ai_chat-Tabellen befreien

Produktionsbug: Plugin ai_assistant war migration_failed/inactive, weil
Migration 0003 ALTER TABLE ai_chat_sessions ausfuehrte — die Tabelle wurde
von Alembic 0137 (2026-08-21, Umstieg auf comm-Tabellen) gedroppt. Bei jedem
Container-Start crashte die Migration und deaktivierte das Plugin
(KI-Chat und /api/v1/ai/* lieferten 403).

Fix:
- 0003: ai_chat_sessions-Statements entfernt, nur ai_chat_folders behalten
- 0001: Ghost-Tabellen ai_chat_sessions/ai_chat_messages entfernt
  (frische Installs duerfen sie nicht rekreieren — Schema-Drift)
- 0002: ai_chat_attachments + toter folder_id-ALTER entfernt,
  nur ai_chat_folders behalten

Runner skipt getrackte Migrationen per Dateiname (kein Hash-Check),
Prod-Risiko null; 0003 laeuft beim naechsten Start sauber durch und
aktiviert das Plugin wieder.

app/plugins/builtins/ai_assistant/migrations/0001_initial.sql

@@ -1,4 +1,7 @@
 -- AI Assistant plugin initial migration
+-- FIX 2026-09-16: ai_chat_sessions/ai_chat_messages removed — AI chat moved
+-- to comm_conversations/comm_messages (Alembic 0137 dropped these tables).
+-- Fresh installs must NOT recreate the ghost tables.
 
 CREATE TABLE IF NOT EXISTS ai_providers (
     id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
@@ -68,35 +71,3 @@ CREATE TABLE IF NOT EXISTS ai_agents (
     deleted_at TIMESTAMPTZ
 );
 CREATE INDEX IF NOT EXISTS ix_ai_agents_tenant ON ai_agents(tenant_id);
-
-CREATE TABLE IF NOT EXISTS ai_chat_sessions (
-    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
-    user_id UUID NOT NULL,
-    agent_id UUID REFERENCES ai_agents(id) ON DELETE SET NULL,
-    title VARCHAR(255) NOT NULL DEFAULT 'Neuer Chat',
-    is_pinned BOOLEAN NOT NULL DEFAULT FALSE,
-    is_sidebar BOOLEAN NOT NULL DEFAULT FALSE,
-    tenant_id UUID NOT NULL,
-    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
-    updated_at TIMESTAMPTZ NOT NULL DEFAULT now(),
-    deleted_at TIMESTAMPTZ
-);
-CREATE INDEX IF NOT EXISTS ix_ai_sessions_user ON ai_chat_sessions(user_id);
-CREATE INDEX IF NOT EXISTS ix_ai_sessions_tenant ON ai_chat_sessions(tenant_id);
-
-CREATE TABLE IF NOT EXISTS ai_chat_messages (
-    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
-    session_id UUID NOT NULL REFERENCES ai_chat_sessions(id) ON DELETE CASCADE,
-    role VARCHAR(20) NOT NULL,
-    content TEXT NOT NULL DEFAULT '',
-    tool_calls JSONB,
-    tool_results JSONB,
-    tokens INTEGER NOT NULL DEFAULT 0,
-    model_used VARCHAR(200) NOT NULL DEFAULT '',
-    tenant_id UUID NOT NULL,
-    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
-    updated_at TIMESTAMPTZ NOT NULL DEFAULT now(),
-    deleted_at TIMESTAMPTZ
-);
-CREATE INDEX IF NOT EXISTS ix_ai_messages_session ON ai_chat_messages(session_id);
-CREATE INDEX IF NOT EXISTS ix_ai_messages_tenant ON ai_chat_messages(tenant_id);