Fix SEC-12: reduce default token expiry to 7 days

- Change default_expiry from 720h (30 days) to 168h (7 days)
  in dist/mcias.conf.example and dist/mcias.conf.docker.example
- Update man page, ARCHITECTURE.md, and config.go comment
- Max ceiling validation remains at 30 days (unchanged)

Security: Shorter default token lifetime reduces the window of
exposure if a token is leaked. 7 days balances convenience and
security for a personal SSO. The 30-day max ceiling is preserved
so operators can still override if needed.

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

internal/config/config.go

@@ -75,7 +75,7 @@ type MasterKeyConfig struct {
 }
 
 // duration is a wrapper around time.Duration that supports TOML string parsing
-// (e.g. "720h", "8h").
+// (e.g. "168h", "8h").
 type duration struct {
 	time.Duration
 }