refactor: minor TigerStyle cleanups

- export_serving.py: move `import re` to module level — was imported
  inside a loop body on every iteration
- sitemap.py: add comment documenting that the in-memory TTL cache is
  process-local (valid for single-worker deployment, Dockerfile --workers 1)
- playtomic_availability.py: use `or "10"` fallback for
  CIRCUIT_BREAKER_THRESHOLD env var to handle empty-string case

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/padelnomics/sitemap.py

@@ -6,6 +6,9 @@ from quart import Response
 
 from .core import fetch_all
 
+# Process-local cache — valid for the current single-Hypercorn-worker deployment
+# (Dockerfile: `--workers 1`). If worker count increases, replace with a
+# DB-backed cache (e.g. a single-row SQLite table with an expires_at column).
 _cache_xml: str = ""
 _cache_timestamp: float = 0.0
 CACHE_TTL_SECONDS: int = 3600  # 1 hour