feat: add conversation persistence and context management layer

- SQLAlchemy async database layer (SQLite dev, PostgreSQL prod)
- Conversation and Message models with UUID primary keys
- Token counting utilities using litellm
- Context summarization at 80% token threshold
- REST API endpoints for multi-turn conversations
- 19 conversation tests, 6 token tests (176 total passing)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

webber-api/src/domains/router.py

@@ -8,6 +8,7 @@ from fastapi import APIRouter
 
 from src.domains.health.router import router as health_router
 from src.domains.agents.router import router as agents_router
+from src.domains.conversations.router import router as conversations_router
 
 # from src.domains.auth.router import router as auth_router
 # from src.domains.tools.router import router as tools_router
@@ -20,6 +21,9 @@ root_router.include_router(health_router)
 # Agents domain (prefix defined in router)
 root_router.include_router(agents_router)
 
+# Conversations domain (prefix defined in router)
+root_router.include_router(conversations_router)
+
 # Auth domain
 # root_router.include_router(auth_router, prefix="/auth", tags=["Auth"])