feat: add secondary school support with KS4 data and metric tooltips

- Backend: replace INNER JOIN ks2 with UNION ALL (ks2 + ks4) so primary
  and secondary schools both appear in the main DataFrame
- Backend: add /api/national-averages endpoint computing means from live
  data, replacing the hardcoded NATIONAL_AVG constant on the frontend
- Backend: add phase filter param to /api/schools; return phases from
  /api/filters; fix hardcoded "phase": "Primary" in school detail endpoint
- Backend: add KS4 metric definitions (Attainment 8, Progress 8, EBacc,
  English & Maths pass rates) to METRIC_DEFINITIONS and RANKING_COLUMNS
- Frontend: SchoolDetailView is now phase-aware — secondary schools show
  a GCSE Results section (Att8, P8, E&M, EBacc) instead of SATs; phonics
  tab hidden for secondary; admissions says Year 7 instead of Year 3;
  history table shows KS4 columns; chart datasets switch for secondary
- Frontend: new MetricTooltip component (CSS-only ⓘ icon) backed by
  METRIC_EXPLANATIONS — added to RWM, GPS, SEN, EAL, IDACI, progress
  scores and all KS4 metrics throughout SchoolDetailView and SchoolCard
- Frontend: METRIC_EXPLANATIONS extended with KS4 terms (Attainment 8,
  Progress 8, EBacc) and previously missing terms (SEN, EHCP, EAL, IDACI)
- Frontend: SchoolCard expands "RWM" to "Reading, Writing & Maths" and
  shows Attainment 8 / English & Maths Grade 4+ for secondary schools
- Frontend: FilterBar adds Phase dropdown (Primary / Secondary / All-through)
- Frontend: HomeView hero copy updated; compact list shows phase-aware metric
- Global metadata updated to remove "primary only" framing

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

backend/schemas.py

@@ -401,6 +401,70 @@ METRIC_DEFINITIONS = {
         "type": "score",
         "category": "trends",
     },
+    # ── GCSE Performance (KS4) ────────────────────────────────────────────
+    "attainment_8_score": {
+        "name": "Attainment 8",
+        "short_name": "Att 8",
+        "description": "Average grade across a pupil's best 8 GCSEs including English and Maths",
+        "type": "score",
+        "category": "gcse",
+    },
+    "progress_8_score": {
+        "name": "Progress 8",
+        "short_name": "P8",
+        "description": "Progress from KS2 baseline to GCSE relative to similar pupils nationally (0 = national average)",
+        "type": "score",
+        "category": "gcse",
+    },
+    "english_maths_standard_pass_pct": {
+        "name": "English & Maths Grade 4+",
+        "short_name": "E&M 4+",
+        "description": "% of pupils achieving grade 4 (standard pass) or above in both English and Maths",
+        "type": "percentage",
+        "category": "gcse",
+    },
+    "english_maths_strong_pass_pct": {
+        "name": "English & Maths Grade 5+",
+        "short_name": "E&M 5+",
+        "description": "% of pupils achieving grade 5 (strong pass) or above in both English and Maths",
+        "type": "percentage",
+        "category": "gcse",
+    },
+    "ebacc_entry_pct": {
+        "name": "EBacc Entry %",
+        "short_name": "EBacc Entry",
+        "description": "% of pupils entered for the English Baccalaureate (English, Maths, Sciences, Languages, Humanities)",
+        "type": "percentage",
+        "category": "gcse",
+    },
+    "ebacc_standard_pass_pct": {
+        "name": "EBacc Grade 4+",
+        "short_name": "EBacc 4+",
+        "description": "% of pupils achieving grade 4+ across all EBacc subjects",
+        "type": "percentage",
+        "category": "gcse",
+    },
+    "ebacc_strong_pass_pct": {
+        "name": "EBacc Grade 5+",
+        "short_name": "EBacc 5+",
+        "description": "% of pupils achieving grade 5+ across all EBacc subjects",
+        "type": "percentage",
+        "category": "gcse",
+    },
+    "ebacc_avg_score": {
+        "name": "EBacc Average Score",
+        "short_name": "EBacc Avg",
+        "description": "Average points score across EBacc subjects",
+        "type": "score",
+        "category": "gcse",
+    },
+    "gcse_grade_91_pct": {
+        "name": "GCSE Grade 9–1 %",
+        "short_name": "GCSE 9–1",
+        "description": "% of GCSE entries achieving a grade 9 to 1",
+        "type": "percentage",
+        "category": "gcse",
+    },
 }
 
 # Ranking columns to include in rankings response
@@ -456,6 +520,16 @@ RANKING_COLUMNS = [
     "rwm_expected_3yr_pct",
     "reading_avg_3yr",
     "maths_avg_3yr",
+    # GCSE (KS4)
+    "attainment_8_score",
+    "progress_8_score",
+    "english_maths_standard_pass_pct",
+    "english_maths_strong_pass_pct",
+    "ebacc_entry_pct",
+    "ebacc_standard_pass_pct",
+    "ebacc_strong_pass_pct",
+    "ebacc_avg_score",
+    "gcse_grade_91_pct",
 ]
 
 # School listing columns