fix: MemoryManager kwarg 'llm' -> 'llm_router'; fix alembic script_location

- main.py: MemoryManager(pool=pool, llm=...) -> llm_router=...
  Class signature is __init__(self, pool, llm_router=None).

- alembic.ini: script_location = migrations -> agent_service/migrations
  When alembic runs from WORKDIR /app inside the container, 'migrations'
  resolves to /app/migrations (missing). Correct path is
  /app/agent_service/migrations where versions/ actually lives.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

agent_service/main.py

@@ -105,7 +105,7 @@ async def lifespan(app: FastAPI):
     # 5. Memory manager
     try:
         from .memory.memory_manager import MemoryManager
-        memory_mgr = MemoryManager(pool=pool, llm=llm_router) if pool else None
+        memory_mgr = MemoryManager(pool=pool, llm_router=llm_router) if pool else None
     except Exception as exc:
         logger.error('Failed to init memory manager: %s', exc)
         memory_mgr = None

agent_service/migrations/alembic.ini

@@ -1,5 +1,5 @@
 [alembic]
-script_location = migrations
+script_location = agent_service/migrations
 prepend_sys_path = .
 version_path_separator = os
 sqlalchemy.url = postgresql+asyncpg://%(POSTGRES_USER)s:%(POSTGRES_PASSWORD)s@%(POSTGRES_HOST)s:%(POSTGRES_PORT)s/%(POSTGRES_DB)s