feat(memory): add WritePipeline and MemoryService facade

Introduce a layered pipeline architecture for the memory write flow:
- WritePipeline: orchestrates preprocess → extract → store → cluster → summarize
  with deadlock retry, resource cleanup, and pilot-run support
- MemoryService: facade that delegates to WritePipeline, placeholder methods
  for read/forget/reflect
- BearLogger: structured step-level logging with perf threshold alerts
- Shadow pipeline integration in MemoryAgentService (env-gated pilot run)

Also includes:
- Fix deprecated SQLAlchemy declarative_base import
- Extend Neo4j Entity fulltext index to cover description and aliases
- Migrate Pydantic schemas to v2 (ConfigDict, field_validator)

api/app/db.py

@@ -2,8 +2,7 @@ import os
 from contextlib import contextmanager
 from typing import Generator
 from sqlalchemy import create_engine
-from sqlalchemy.orm import Session, sessionmaker
-from sqlalchemy.ext.declarative import declarative_base
+from sqlalchemy.orm import Session, sessionmaker, declarative_base
 from app.core.config import settings
 
 SQLALCHEMY_DATABASE_URL = f"postgresql://{settings.DB_USER}:{settings.DB_PASSWORD}@{settings.DB_HOST}:{settings.DB_PORT}/{settings.DB_NAME}"