feat(templates): add pagination with total counts to catalog and campaign picker

Test Catalog's pagination showed only 'Page N' with no way to know
how many pages existed, and the campaign 'Add Test' modal's template
picker had no pagination at all — with 2800+ templates in the
catalog, its flat 100-row fetch made most templates unreachable
unless a search happened to match. Adds GET /test-templates/count
(open to any authenticated user, mirrors the list endpoint's filters)
and wires real 'Page N of M' + total-available counts into both.

backend/app/routers/test_templates.py

@@ -58,6 +58,7 @@ from app.services.audit_service import log_action
 # Import  from app.services.test_template_service
 from app.services.test_template_service import (
     bulk_activate,
+    count_templates,
     get_template_or_raise,
     get_template_stats,
     list_templates,
@@ -156,6 +157,35 @@ def _list_templates_handler(
     )
 
 
+# ---------------------------------------------------------------------------
+# GET /test-templates/count  —  total matching a filter, for pagination
+# ---------------------------------------------------------------------------
+
+
+@router.get("/count")
+def _count_templates_handler(
+    source: Optional[str] = Query(None),
+    platform: Optional[str] = Query(None),
+    severity: Optional[str] = Query(None),
+    mitre_technique_id: Optional[str] = Query(None),
+    search: Optional[str] = Query(None),
+    is_active: Optional[bool] = Query(None),
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+) -> dict:
+    """Return the total count of templates matching the same filters as the
+    list endpoint, so the catalog UI can show a true page count."""
+    return {"total": count_templates(
+        db,
+        source=source,
+        platform=platform,
+        severity=severity,
+        mitre_technique_id=mitre_technique_id,
+        search=search,
+        is_active=is_active,
+    )}
+
+
 # ---------------------------------------------------------------------------
 # GET /test-templates/stats  —  catalog statistics (admin)
 # ---------------------------------------------------------------------------