Add Parser Quality page: Live Event Sampler, Field Population Rate, Parser Test Runner

- New /api/quality router with three endpoints:
  sample-events: pull raw events from a source via PowerQuery
  field-population: measure % of events with each SDL field populated;
    surfaces dataSource.name correctly (100% when filtered by it) and
    returns fields_seen_in_sample so you can see what IS being extracted
  test-parser: converts SDL \$field=pattern\$ format strings to Python
    named-group regex and tests against a pasted raw log line
- New "Parser Quality" nav item and page with all three tools
- Home page card added for Parser Quality
- Field population UI shows per-field colour-coded progress bars plus
  a chip list of fields actually present in the sample

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

backend/main.py

@@ -1,7 +1,7 @@
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 from db import engine, Base
-from routers import coverage, ingest, settings
+from routers import coverage, ingest, settings, quality
 
 Base.metadata.create_all(bind=engine)
 
@@ -18,6 +18,7 @@ app.add_middleware(
 app.include_router(coverage.router, prefix="/api/coverage", tags=["Coverage"])
 app.include_router(ingest.router,   prefix="/api/ingest",   tags=["Ingest"])
 app.include_router(settings.router, prefix="/api/settings", tags=["Settings"])
+app.include_router(quality.router,  prefix="/api/quality",  tags=["Quality"])
 
 
 @app.get("/health")