chore(F.4e): remove env-based site loading, unify OAuth Clients, update docs

- Remove env-based site discovery from SiteManager and ProjectManager
- Unify admin and user OAuth Clients into single role-based page
- Update all documentation to reflect DB-based site management
- Clean up env.example, README, ARCHITECTURE docs
- Improve Projects page empty state with link to My Sites
- Remove obsolete env discovery tests

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

core/templates/dashboard/base.html

@@ -138,6 +138,9 @@
                 ('services', t.get('services', 'Services'), 'M19 11H5m14 0a2 2 0 012 2v6a2 2 0 01-2 2H5a2 2 0 01-2-2v-6a2 2 0 012-2m14 0V9a2 2 0 00-2-2M5 11V9a2 2 0 012-2m0 0V5a2 2 0 012-2h6a2 2 0 012 2v2M7 7h10', '/dashboard/services'),
                 ('connect', t.get('connect', 'Connect'), 'M13.828 10.172a4 4 0 00-5.656 0l-4 4a4 4 0 105.656
                 5.656l1.102-1.101m-.758-4.899a4 4 0 005.656 0l4-4a4 4 0 00-5.656-5.656l-1.1 1.1', '/dashboard/connect'),
+                ('oauth_clients', t.oauth_clients, 'M9 12l2 2 4-4m5.618-4.016A11.955 11.955 0 0112 2.944a11.955 11.955 0
+                01-8.618 3.04A12.02 12.02 0 003 9c0 5.591 3.824 10.29 9 11.622 5.176-1.332 9-6.03 9-11.622
+                0-1.042-.133-2.052-.382-3.016z', '/dashboard/oauth-clients'),
                 ] %}
 
                 {# ── Admin-only nav items ── #}
@@ -146,9 +149,6 @@
                 '/dashboard/projects'),
                 ('api_keys', t.api_keys, 'M15 7a2 2 0 012 2m4 0a6 6 0 01-7.743 5.743L11 17H9v2H7v2H4a1 1 0
                 01-1-1v-2.586a1 1 0 01.293-.707l5.964-5.964A6 6 0 1121 9z', '/dashboard/api-keys'),
-                ('oauth_clients', t.oauth_clients, 'M9 12l2 2 4-4m5.618-4.016A11.955 11.955 0 0112 2.944a11.955 11.955 0
-                01-8.618 3.04A12.02 12.02 0 003 9c0 5.591 3.824 10.29 9 11.622 5.176-1.332 9-6.03 9-11.622
-                0-1.042-.133-2.052-.382-3.016z', '/dashboard/oauth-clients'),
                 ('audit_logs', t.audit_logs, 'M9 5H7a2 2 0 00-2 2v12a2 2 0 002 2h10a2 2 0 002-2V7a2 2 0 00-2-2h-2M9 5a2
                 2 0 002 2h2a2 2 0 002-2M9 5a2 2 0 012-2h2a2 2 0 012 2m-3 7h3m-3 4h3m-6-4h.01M9 16h.01',
                 '/dashboard/audit-logs'),