Add Settings page with .env manager

- Sidebar: ⚙ Settings link pinned to bottom of nav
- Settings page: view all config keys (secrets masked), edit and save directly to .env
- Show/hide toggle for secret fields (tokens, keys)
- First-time setup banner with cp .env.example .env instructions when .env is missing
- Manual setup section with step-by-step terminal commands and where to find each credential
- New .env.example template with comments for all required variables
- Backend: GET/POST /api/settings/config router reads/writes mounted .env file
- docker-compose: mounts .env into backend container at /app/.env for write access

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
+from routers import coverage, ingest, settings
 
 Base.metadata.create_all(bind=engine)
 
@@ -16,7 +16,8 @@ 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(ingest.router,   prefix="/api/ingest",   tags=["Ingest"])
+app.include_router(settings.router, prefix="/api/settings", tags=["Settings"])
 
 
 @app.get("/health")