[enhancement] Support topics and entities as filter in Summary Views

Summary

Summary Views currently support only four keys for filters and group_by: user_id, namespace, session_id, and memory_type. This proposal requests adding topics and entities as supported keys, enabling domain (topic) specific summary views that are far more useful for real-world applications.

Motivation

In a wealth advisor demo application, we have long-term memories tagged with topics like investment, retirement, cash flow. Currently, the only way to create a focused summary is to summarize all memories for a user and depend on LLM prompt property control the output. There's no way to pre-filter at the data level.

With topics and entities support, we could create summary views like:

{
  "name": "Investment Strategy Summary",
  "source": "long_term",
  "group_by": ["user_id"],
  //"prompt":"",
  "filters": { "topics": { "any": ["investment", "portfolio", "retirement"] } }
}

{
  "name": "Per-Topic User Summary",
  "source": "long_term",
  "group_by": ["user_id", "topics"]
}

{
  "name": "Entity-Focused Summary",
  "source": "long_term",
  "group_by": ["user_id"],
  "filters": { "entities": { "any": ["Vanguard"] } }
}

These would allow applications to generate targeted summaries like "What does the system know about this user's retirement planning?" or "Summarize all interactions involving Vanguard funds."

No files, tests, or entry points are named. Start by locating the Summary Views implementation and its existing filters and group_by handling, then trace the memory fields used for topics and entities; done means both keys are accepted for filtering and grouping with the examples in the issue covered by tests.