Add user memory feature for cross-conversation recall

Give the LangChain agent a save_user_memory tool so users can ask it to
remember preferences and personal facts. Memories are stored per-user in
a new user_memories table and injected into the system prompt on each
conversation turn.

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

blueprints/conversation/memory.py

@@ -0,0 +1,19 @@
+from .models import UserMemory
+
+
+async def get_memories_for_user(user_id: str) -> list[str]:
+    """Return all memory content strings for a user, ordered by most recently updated."""
+    memories = await UserMemory.filter(user_id=user_id).order_by("-updated_at")
+    return [m.content for m in memories]
+
+
+async def save_memory(user_id: str, content: str) -> str:
+    """Save a new memory or touch an existing one (exact-match dedup)."""
+    existing = await UserMemory.filter(user_id=user_id, content=content).first()
+    if existing:
+        existing.updated_at = None  # auto_now=True will refresh it on save
+        await existing.save(update_fields=["updated_at"])
+        return "Memory already exists (refreshed)."
+
+    await UserMemory.create(user_id=user_id, content=content)
+    return "Memory saved."