feat: wire Typesense search into backend, fix sync performance data bug

sync_typesense.py:
- Fix query string replacement: was matching 'ST_X(l.geom) as lng' but
  QUERY_BASE uses 'l.longitude as lng' — KS2/KS4 lateral joins were
  silently dropped on every sync run

backend:
- Add typesense_url/typesense_api_key settings to config.py
- Add search_schools_typesense() to data_loader.py — queries Typesense
  'schools' alias, returns URNs in relevance order with typo tolerance;
  falls back to empty list if Typesense is unavailable
- /api/schools: replace pandas str.contains with Typesense search;
  results are filtered from the DataFrame and returned in relevance order;
  graceful fallback to substring match if Typesense is down

requirements.txt: add typesense==0.21.0, numpy==1.26.4

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

backend/config.py

@@ -38,6 +38,10 @@ class Settings(BaseSettings):
     rate_limit_burst: int = 10  # Allow burst of requests
     max_request_size: int = 1024 * 1024  # 1MB max request size
 
+    # Typesense
+    typesense_url: str = "http://localhost:8108"
+    typesense_api_key: str = ""
+
     # Analytics
     ga_measurement_id: Optional[str] = "G-J0PCVT14NY"  # Google Analytics 4 Measurement ID