feat(api): decide which nearby schools a page may offer

Hard filters encode claims the section may not make — a selective school is
not an alternative to a non-selective one, a special school is not comparable
to a mainstream one, a Girls school is not an option for a Boys school's
reader — so they never relax. Soft preferences describe closeness of fit, so
they relax across three tiers, and only far enough to reach three; the
remaining slots up to six fill from the tiers already opened.

PHASE_GROUPS moves to schemas.py so this module can share it without
importing app, which would be a cycle.

_mask() exists because Series.apply on an empty Series returns a DataFrame,
and using that as a mask drops every column — so the next lookup raises
KeyError instead of yielding no rows. A special school with no special school
near it empties the frame at the provision filter, which is the ordinary case
for most special schools, so this was a crash on a common path rather than an
edge case.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/app.py

@@ -40,20 +40,12 @@ from .data_loader import (
 from .data_loader import get_data_info as get_db_info
 from . import flags
 from .places import build_place_index, build_place_registry, places_for_urn
-from .schemas import METRIC_DEFINITIONS, RANKING_COLUMNS, SCHOOL_COLUMNS
+from .schemas import METRIC_DEFINITIONS, PHASE_GROUPS, RANKING_COLUMNS, SCHOOL_COLUMNS
 from .utils import clean_for_json, convert_to_native
 
 # Values to exclude from filter dropdowns (empty strings, non-applicable labels)
 EXCLUDED_FILTER_VALUES = {"", "Not applicable", "Does not apply"}
 
-# Maps user-facing phase filter values to the GIAS PhaseOfEducation values they include.
-# All-through schools appear in both primary and secondary results.
-PHASE_GROUPS: dict[str, set[str]] = {
-    "primary": {"primary", "middle deemed primary", "all-through"},
-    "secondary": {"secondary", "middle deemed secondary", "all-through", "16 plus"},
-    "all-through": {"all-through"},
-}
-
 # Must match SITE_URL in nextjs-app/lib/site.ts. The apex 301s to www, and a
 # sitemap <loc> that redirects wastes a crawl on every URL it lists.
 BASE_URL = "https://www.schoolcompare.co.uk"