Fix runtime bugs, implement service layer, and overhaul documentation

Runtime fixes:
- Fix env var mismatch (CERTCTL_DB_URL → CERTCTL_DATABASE_URL)
- Fix table name mismatches (certificates → managed_certificates, notifications → notification_events)
- Add renewal_policy_id to certificate queries
- Remove non-existent created_at from notification queries
- Add env var fallback for agent CLI flags
- Graceful degradation for missing notifiers/issuers in demo mode
- Copy web/ directory in Dockerfile for dashboard serving

Service layer:
- Implement handler-service interface pattern across all services
- Wire up certificate, agent, job, policy, team, owner, audit, notification services

Documentation:
- Add concepts.md: beginner-friendly guide to TLS, CAs, private keys
- Rewrite quickstart.md with accurate API examples matching actual handlers
- Add demo-advanced.md: interactive demo with cert issuance and automated script
- Update architecture.md with correct table names and connector interfaces
- Update connectors.md to match actual Go interface signatures
- Update demo-guide.md with cross-references to new docs

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

Dockerfile

@@ -30,6 +30,7 @@ WORKDIR /app
 
 COPY --from=builder /app/bin/server .
 COPY --chown=certctl:certctl migrations/ ./migrations/
+COPY --chown=certctl:certctl web/ ./web/
 
 RUN chown -R certctl:certctl /app