step-6: add memory models and migration (models.py, alembic)

- User.encryption_key: per-user Fernet key generated on registration
- MemoryCore: encrypted key/value preferences
- MemoryAssociative: encrypted semantic memory + pgvector(1536) embedding
- MemoryEpisodic: encrypted session summaries
- MemoryProactive: encrypted behavioral patterns with confidence score
- Migration 004: enables pgvector extension, creates all 4 tables + ivfflat index
- auth.py register: generates Fernet key for new users
- 8 unit tests pass (SQLite in-memory, JSON embedding fallback)

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

app/api/routes/auth.py

@@ -13,6 +13,7 @@ import uuid
 from datetime import datetime, timedelta, timezone
 
 import bcrypt
+from cryptography.fernet import Fernet
 from fastapi import APIRouter, Depends, HTTPException, status
 from jose import jwt
 from pydantic import BaseModel
@@ -94,6 +95,7 @@ async def register(
         email=body.email,
         password_hash=_hash_password(body.password),
         tier="free",
+        encryption_key=Fernet.generate_key().decode(),
     )
     db.add(user)
     await db.flush()  # get user.id without committing