fix: add precondition assertions in extract pipeline

Assert landing_dir.is_dir() and year_month format (YYYY/MM) at the
entry point of each extract function — turning silent wrong-path bugs
into immediate AssertionError with a descriptive message.

Files changed:
- playtomic_availability.py: assert in _load_tenant_ids(), extract(),
  extract_recheck()
- eurostat.py: assert in extract()

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

extract/padelnomics_extract/src/padelnomics_extract/eurostat.py

@@ -180,6 +180,8 @@ def extract(
     session: niquests.Session,
 ) -> dict:
     """Fetch all Eurostat datasets. Returns run metrics."""
+    assert landing_dir.is_dir(), f"landing_dir must exist: {landing_dir}"
+    assert "/" in year_month and len(year_month) == 7, f"year_month must be YYYY/MM: {year_month!r}"
     year, month = year_month.split("/")
     files_written = 0
     files_skipped = 0

extract/padelnomics_extract/src/padelnomics_extract/playtomic_availability.py

@@ -59,6 +59,7 @@ _thread_local = threading.local()
 
 def _load_tenant_ids(landing_dir: Path) -> list[str]:
     """Read tenant IDs from the most recent tenants.json.gz file."""
+    assert landing_dir.is_dir(), f"landing_dir must exist: {landing_dir}"
     playtomic_dir = landing_dir / "playtomic"
     if not playtomic_dir.exists():
         return []
@@ -243,6 +244,8 @@ def extract(
     session: niquests.Session,
 ) -> dict:
     """Fetch next-day availability for all known Playtomic venues."""
+    assert landing_dir.is_dir(), f"landing_dir must exist: {landing_dir}"
+    assert "/" in year_month and len(year_month) == 7, f"year_month must be YYYY/MM: {year_month!r}"
     tenant_ids = _load_tenant_ids(landing_dir)
     if not tenant_ids:
         logger.warning("No tenant IDs found — run extract-playtomic-tenants first")
@@ -385,6 +388,8 @@ def extract_recheck(
     session: niquests.Session,
 ) -> dict:
     """Re-query venues with slots starting soon for accurate occupancy data."""
+    assert landing_dir.is_dir(), f"landing_dir must exist: {landing_dir}"
+    assert "/" in year_month and len(year_month) == 7, f"year_month must be YYYY/MM: {year_month!r}"
     now = datetime.now(UTC)
     target_date = now.strftime("%Y-%m-%d")