Use subquery for common third party filter

Replace the two-step ID-materializing pattern (fetch IDs in Go, pass
as ANY(@ids)) with an IN-subquery that keeps the filtering entirely
in the database and eliminates an extra round trip. Remove the now
unused LoadIDsByCommonThirdPartyID and its service wrapper. Update
the coredata rule to clarify that subqueries for filtering are OK.

Signed-off-by: Émile Ré <emile@probo.com>

.cursor/rules/go-coredata-load-naming.mdc

@@ -58,15 +58,20 @@ func (ds *Things) LoadByParentID(
 ) error {
 ```
 
-# No cross-entity JOINs
+# Cross-entity table references
 
-Each entity file in `pkg/coredata` queries only its own table. When data from multiple entities is needed, the caller orchestrates separate calls.
+Each entity file in `pkg/coredata` queries its own table. Never JOIN two
+entity tables to **return columns from both** — the caller orchestrates
+separate calls instead.
 
-- Never JOIN two entity tables inside an entity method.
-- Never return a raw ID belonging to a different entity — return the full entity and let the caller read the foreign key field.
+**Subqueries for filtering are OK.** When the only purpose of the other
+table is to narrow a `WHERE` clause (e.g. `IN (SELECT id FROM ...)`),
+keep it in the query rather than materializing IDs in Go and passing
+them as an `ANY(@ids)` parameter. A subquery keeps the filtering in the
+database and eliminates an extra round trip.
 
 ```go
-// BAD — cross-entity JOIN inside DetectedTrackers
+// BAD — cross-entity JOIN that returns columns from both tables
 q := `
 SELECT ctpd.common_third_party_id
 FROM detected_trackers dt
@@ -80,4 +85,14 @@ filter := coredata.NewCommonThirdPartyDomainFilter(domains)
 var matched coredata.CommonThirdPartyDomains
 err = matched.Load(ctx, conn, 1, filter)
 thirdPartyID := matched[0].CommonThirdPartyID
+
+// GOOD — subquery only used for filtering, no columns returned from it
+q := `
+SELECT id, pattern, tracker_type, ...
+FROM tracker_patterns
+WHERE common_tracker_pattern_id IN (
+    SELECT id FROM common_tracker_patterns
+    WHERE common_third_party_id = @filter_common_third_party_id
+)
+`
 ```