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>

nextjs-app/components/PerformanceChart.tsx

@@ -6,24 +6,12 @@
 'use client';
 
 import { Line } from 'react-chartjs-2';
-import {
-  Chart as ChartJS,
-  CategoryScale,
-  LinearScale,
-  PointElement,
-  LineElement,
-  Title,
-  Tooltip,
-  Legend,
-  ChartOptions,
-  ChartDataset,
-} from 'chart.js';
+import { ChartOptions, ChartDataset } from 'chart.js';
+import '@/lib/chartSetup';
 import type { SchoolResult } from '@/lib/types';
 import { formatAcademicYear } from '@/lib/utils';
 import styles from './PerformanceChart.module.css';
 
-ChartJS.register(CategoryScale, LinearScale, PointElement, LineElement, Title, Tooltip, Legend);
-
 interface NationalByYear {
   year: number;
   primary: Record<string, number>;