Build tracker identification prompt categories dynamically

Generate the category list from coredata.ThirdPartyCategories() at
runtime instead of hardcoding it in the prompt text. Type the
TrackerIdentification.Category field as coredata.ThirdPartyCategory so
JSON unmarshaling validates values automatically.

Also documents the .txt.tmpl template file naming convention.

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

pkg/coredata/common_third_party.go

@@ -214,6 +214,7 @@ LIMIT 1;
 		if errors.Is(err, pgx.ErrNoRows) {
 			return ErrResourceNotFound
 		}
+
 		return fmt.Errorf("cannot collect common third party by slug: %w", err)
 	}