feat: sitemap hreflang alternates, caching, and lastmod cleanup

Extract sitemap generation to sitemap.py with xhtml:link hreflang
alternates (en/de/x-default) on every URL entry. Add 1-hour in-memory
TTL cache with Cache-Control header. Include supplier pages in both
languages (were EN-only). Drop misleading "today" lastmod from static
pages.

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

web/tests/conftest.py

@@ -12,10 +12,12 @@ from unittest.mock import AsyncMock, patch
 
 import aiosqlite
 import pytest
-from padelnomics import core
 from padelnomics.app import create_app
 from padelnomics.migrations.migrate import migrate
 
+from padelnomics import core
+from padelnomics import sitemap as sitemap_mod
+
 _SCHEMA_CACHE = None
 
 
@@ -56,6 +58,9 @@ async def db():
 
     original_db = core._db
     core._db = conn
+    # Clear sitemap cache so tests see fresh DB state
+    sitemap_mod._cache_xml = ""
+    sitemap_mod._cache_timestamp = 0.0
 
     yield conn
 
@@ -147,6 +152,7 @@ def create_subscription(db):
 async def scenario(db, test_user):
     """User scenario with valid planner state for PDF generation."""
     import json
+
     from padelnomics.planner.calculator import validate_state
     state = validate_state({"dblCourts": 4, "sglCourts": 2})
     now = datetime.utcnow().isoformat()