feat: add AI Digital Employee agent orchestrator with pipeline tracking

- New AgentPipeline model with JSONB pipeline_data for stages/leads/summary
- AgentOrchestrator service chains DiscoveryService search→analyze→outreach→auto-save
- 3 new API endpoints: POST /agent/start, GET /agent/pipelines, GET /agent/{id}
- Full Agent dashboard Vue component with stats, pipeline grid, leads table, outreach preview
- Sidebar redesigned with AI Agent as primary entry point
- Updated PROGRESS.md, AGENTS.md, DATABASE_SCHEMA.md with latest state

backend/app/models/__init__.py

@@ -23,6 +23,7 @@ from .credit_package import CreditPackage, SubscriptionPlan
 from .user_credit import UserCredit
 from .credit_consumption import CreditConsumption
 from .credit_purchase import CreditPurchase
+from .agent_pipeline import AgentPipeline
 
 __all__ = [
     "User", "Product",
@@ -45,4 +46,5 @@ __all__ = [
     "UserCredit",
     "CreditConsumption",
     "CreditPurchase",
+    "AgentPipeline",
 ]