feat(4636): rule pull-through per surface, and per-rule usage with confirmed lessons on list_rules

retrieval_telemetry gains rule_usage.by_source, the twin of usage.by_source:
distinct rules each surface showed, how many an agent opened, and a ratio
that is null on a bulk delivery. list_rules(with_usage=True, days=N) adds
each rule's usage, windowed, and how many lessons are confirmed instances
of it, so linked and unlinked rules can be compared through MCP.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/grimoire/services/rule_usage.py

@@ -59,8 +59,9 @@ as short and as slow-moving as possible.
 from __future__ import annotations
 
 import logging
+from datetime import datetime
 
-from sqlalchemy import case, func, select
+from sqlalchemy import case, func, select, true
 
 from grimoire.models import async_session
 from grimoire.models.base import iso
@@ -333,13 +334,18 @@ def empty_rule_usage() -> dict:
     }
 
 
-async def usage_for_rules(rule_ids: list[int]) -> dict[int, dict]:
+async def usage_for_rules(
+    rule_ids: list[int], since: datetime | None = None,
+) -> dict[int, dict]:
     """Aggregate usage for a set of rules: {rule_id: {counts + timestamps}}.
 
     One GROUP BY for the whole page rather than a query per row — this feeds a
     list view, so the per-row shape would be N+1 by construction. Rules with no
     events come back with `empty_rule_usage()`, so the caller never has to tell
     "no events" from "not in the result".
+
+    `since` limits the counts to events at or after it, so two windows can be
+    compared; without it they cover everything recorded.
     """
     ids = [int(r) for r in rule_ids]
     out: dict[int, dict] = {rid: empty_rule_usage() for rid in ids}
@@ -370,6 +376,10 @@ async def usage_for_rules(rule_ids: list[int]) -> dict[int, dict]:
                         ambient,
                     )
                     .where(RuleUsageEvent.rule_id.in_(ids))
+                    .where(
+                        RuleUsageEvent.created_at >= since
+                        if since is not None else true()
+                    )
                     .group_by(
                         RuleUsageEvent.rule_id,
                         RuleUsageEvent.event,