fix(api): filter by every GIAS phase, not just the grouped ones

/api/schools only recognised primary, secondary and all-through. Any
other phase the search page offers (nursery, 16 plus, middle deemed
primary/secondary) fell through to no filter, so "Nursery" returned the
whole result set, mostly primaries. Ungrouped phases now match exactly,
and an unknown phase returns nothing rather than everything.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app.py

@@ -736,7 +736,7 @@ async def get_schools(
         None, description="Filter by local authority", max_length=100
     ),
     school_type: Optional[str] = Query(None, description="Filter by school type", max_length=100),
-    phase: Optional[str] = Query(None, description="Filter by phase: primary, secondary, all-through", max_length=50),
+    phase: Optional[str] = Query(None, description="Filter by phase: primary or secondary (grouped), or any GIAS phase name (exact)", max_length=50),
     postcode: Optional[str] = Query(None, description="Search near postcode", max_length=10),
     radius: float = Query(5.0, ge=0.1, le=5, description="Search radius in miles"),
     page: int = Query(1, ge=1, le=1000, description="Page number"),
@@ -771,11 +771,13 @@ async def get_schools(
 
     # Phase filter — uses PHASE_GROUPS so all-through/middle schools appear
     # in the correct phase(s) rather than being invisible to both filters.
+    # Any other GIAS phase (nursery, 16 plus, middle deemed ...) is an exact
+    # match. It must never fall through to no filter: the search page offers
+    # every phase, and "Nursery" used to return the whole result set.
     if phase:
         phase_lower = phase.lower().replace("_", "-")
-        allowed = PHASE_GROUPS.get(phase_lower)
-        if allowed:
-            df_latest = df_latest[df_latest["phase"].str.lower().isin(allowed)]
+        allowed = PHASE_GROUPS.get(phase_lower, {phase_lower})
+        df_latest = df_latest[df_latest["phase"].fillna("").str.lower().isin(allowed)]
 
     # Secondary-specific filters (after phase filter)
     if gender: