fix(data): strip any table alias in missing-column matcher, not just s.

The graceful-degradation fallback keys off the column named in a Postgres
UndefinedColumn error, but the matcher only stripped an `s.` alias. The two
new dim_location columns (county, parliamentary_constituency) are selected
via the `l.` alias and Postgres reports them unquoted as
"column l.county does not exist" — which the old regex failed to match at
all, returning None.

If dim_school is rebuilt (telephone/nursery present) but dim_location is not
yet (county/parliamentary_constituency missing) — plausible since they are
independently-rebuilt dbt models — the fallback branch never matched and
load_school_data_as_dataframe() returned an empty DataFrame, showing zero
schools sitewide instead of degrading those columns to NULL.

Generalise the alias prefix to `\w+\.` and cover the l.-qualified case in
tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/data_loader.py

@@ -339,7 +339,11 @@ _GIAS_CODE_COLUMN_NAMES = (
     "admissions_policy_code",
 )
 
-_MISSING_COLUMN_RE = re.compile(r'column "?(?:s\.)?(\w+)"? does not exist')
+# Strip any single table alias prefix (s., l., p., foi., …) — Postgres reports
+# an undefined *qualified* column unquoted as "column l.county does not exist",
+# so matching only the s. alias would miss dim_location columns (county,
+# parliamentary_constituency) and defeat the graceful-degradation fallback.
+_MISSING_COLUMN_RE = re.compile(r'column "?(?:\w+\.)?(\w+)"? does not exist')
 
 
 def _missing_column_name(exc: Exception) -> Optional[str]:

backend/tests/test_gias_translation.py

@@ -73,6 +73,24 @@ def test_missing_column_name_table_prefixed():
     )
 
 
+def test_missing_column_name_location_alias_prefixed():
+    # dim_location columns are selected via the `l.` alias; Postgres reports a
+    # missing qualified column unquoted (e.g. "column l.county does not exist").
+    # The matcher must strip any alias, not just `s.`, or the county /
+    # parliamentary_constituency fallback never triggers and the whole data
+    # load degrades to an empty DataFrame (zero schools) instead of NULLs.
+    assert (
+        _missing_column_name(_fake_exc("column l.county does not exist"))
+        == "county"
+    )
+    assert (
+        _missing_column_name(
+            _fake_exc("column l.parliamentary_constituency does not exist")
+        )
+        == "parliamentary_constituency"
+    )
+
+
 def test_missing_column_name_no_match_returns_none():
     assert _missing_column_name(_fake_exc("relation \"marts.dim_school\" does not exist")) is None