perf: resolve all P1–P5 performance issues from code review

P1 (backend/data_loader.py): Add load_latest_school_data() which pre-computes
the one-row-per-school latest-year snapshot (groupby, prev-year trend merge)
once at startup instead of on every /api/schools request. get_schools route
now starts from the cached snapshot rather than rebuilding it.

S3 (backend/app.py): Wrap synchronous geocode_single_postcode() call in
asyncio.to_thread() so postcode lookups no longer block the uvicorn event
loop. Admin reload endpoint also uses to_thread for both cache primes.

P2 (nextjs-app/components/HomeView.tsx): Add mapParamsRef guard so switching
back to map view does not re-fetch 500 schools when search params haven't
changed. Reset ref on new searches so fresh data is always fetched.

P3 (nextjs-app/lib/chartSetup.ts): Extract Chart.js registration into a
shared side-effect module. ComparisonChart and PerformanceChart now import
it instead of each calling ChartJS.register() independently.

P4 (backend/database.py): Remove unnecessary db.commit() from the read-only
get_db_session() context manager — saves a DB round-trip on every request.

P5 (backend/database.py): Add pool_recycle=1800 to SQLAlchemy engine to
prevent stale TCP connections from accumulating in long-running processes.

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

backend/database.py

@@ -15,6 +15,7 @@ engine = create_engine(
     pool_size=10,
     max_overflow=20,
     pool_pre_ping=True,
+    pool_recycle=1800,  # recycle connections every 30 min to avoid stale TCP
     echo=False,
 )
 
@@ -34,13 +35,9 @@ def get_db():
 
 @contextmanager
 def get_db_session():
-    """Context manager for non-FastAPI contexts."""
+    """Context manager for non-FastAPI contexts (read-only)."""
     db = SessionLocal()
     try:
         yield db
-        db.commit()
-    except Exception:
-        db.rollback()
-        raise
     finally:
         db.close()