fix: add updated_at columns to ai_proactive migration

- ai_proactive_suggestions: add updated_at TIMESTAMPTZ
- ai_proactive_context_log: add updated_at TIMESTAMPTZ

TenantMixin inherits TimestampMixin which expects both created_at and updated_at.
Missing updated_at caused 500 errors on /ai-proactive/context POST.

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

@@ -16,6 +16,7 @@ CREATE TABLE IF NOT EXISTS ai_proactive_suggestions (
     is_acted_upon BOOLEAN NOT NULL DEFAULT FALSE,
     context_snapshot JSONB NOT NULL DEFAULT '{}',
     created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
     expires_at TIMESTAMPTZ,
     deleted_at TIMESTAMPTZ
 );
@@ -32,6 +33,7 @@ CREATE TABLE IF NOT EXISTS ai_proactive_context_log (
     entity_id UUID,
     entity_data JSONB NOT NULL DEFAULT '{}',
     created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
     deleted_at TIMESTAMPTZ
 );
 CREATE INDEX IF NOT EXISTS ix_context_log_user_time ON ai_proactive_context_log (tenant_id, user_id, created_at DESC);