web: DRY Step 5 and alternatives (partial+macro), centralize start_ctx/owned_set, adopt builder_*

code/web/services/alts_utils.py

@@ -0,0 +1,25 @@
+from __future__ import annotations
+
+from typing import Dict, Tuple
+import time as _t
+
+# Lightweight in-memory TTL cache for alternatives fragments
+_ALTS_CACHE: Dict[Tuple[str, str, bool], Tuple[float, str]] = {}
+_ALTS_TTL_SECONDS = 60.0
+
+
+def get_cached(key: tuple[str, str, bool]) -> str | None:
+    try:
+        ts, html = _ALTS_CACHE.get(key, (0.0, ""))
+        if ts and (_t.time() - ts) < _ALTS_TTL_SECONDS:
+            return html
+    except Exception:
+        return None
+    return None
+
+
+def set_cached(key: tuple[str, str, bool], html: str) -> None:
+    try:
+        _ALTS_CACHE[key] = (_t.time(), html)
+    except Exception:
+        pass