Move SSO clients from config to database

- Add sso_clients table (migration 000010) with client_id, redirect_uri,
  tags (JSON), enabled flag, and audit timestamps
- Add SSOClient model struct and audit events
- Implement DB CRUD with 10 unit tests
- Add REST API: GET/POST/PATCH/DELETE /v1/sso/clients (policy-gated)
- Add gRPC SSOClientService with 5 RPCs (admin-only)
- Add mciasctl sso list/create/get/update/delete commands
- Add web UI admin page at /sso-clients with HTMX create/toggle/delete
- Migrate handleSSOAuthorize and handleSSOTokenExchange to use DB
- Remove SSOConfig, SSOClient struct, lookup methods from config
- Simplify: client_id = service_name for policy evaluation

Security:
- SSO client CRUD is admin-only (policy-gated REST, requireAdmin gRPC)
- redirect_uri must use https:// (validated at DB layer)
- Disabled clients are rejected at both authorize and token exchange
- All mutations write audit events

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/templates/base.html

@@ -15,6 +15,7 @@
       {{if .IsAdmin}}<li><a href="/accounts">Accounts</a></li>
       <li><a href="/audit">Audit</a></li>
       <li><a href="/policies">Policies</a></li>
+      <li><a href="/sso-clients">SSO Clients</a></li>
       <li><a href="/pgcreds">PG Creds</a></li>{{else}}<li><a href="/service-accounts">Service Accounts</a></li>{{end}}
       {{if .ActorName}}<li><a href="/profile">{{.ActorName}}</a></li>{{end}}
       <li><form method="POST" action="/logout" style="margin:0"><button class="btn btn-sm btn-secondary" type="submit">Logout</button></form></li>