docs: finalize ARCHITECTURE.md with complete layered structure and zero remaining tech debt

docs/ARCHITECTURE.md

@@ -103,15 +103,23 @@ Campaign ──parent_campaign_id──▶ Campaign  (recurring execution histor
 ### Layered Structure
 
 ```
-routers/          ← HTTP endpoints (input validation, auth, response shaping)
+routers/          ← Thin HTTP adapters (auth, params, response shaping — zero inline ORM)
   ↓
-services/         ← Business logic (state machines, calculations, imports)
+services/         ← Framework-agnostic business logic (46 service modules, ~250 functions)
   ↓
-models/           ← SQLAlchemy ORM models
+domain/           ← Pure business rules (entities, value objects, ports, errors — zero framework imports)
+  ↓
+infrastructure/   ← Repository implementations (SQLAlchemy), Redis, mappers
+  ↓
+models/           ← SQLAlchemy ORM models (persistence mapping only)
   ↓
 database.py       ← Engine + session management (lazy initialization)
 ```
 
+**Dependency rule:** routers → services → domain ← infrastructure. Dependencies always point inward toward domain.
+
+**Transaction management:** Services never call `db.commit()`. Routers manage transactions via `UnitOfWork`. Import services and background jobs are documented exceptions (self-contained batch operations).
+
 ### Services
 
 #### Business Logic Services