feat(server): follow-ups to scoped mutation locks (#4147) #4307

Follow-ups to #4147, part of #3528. One PR per line, in the order we're working them. Each PR gets its own narrow issue with the details. The earlier long write-up is in this issue's edit history.

• Startup-sweep lock waits and timeouts show up in the lock metrics (fix(server): count startup-sweep lock waits in the lock metrics #4523)
• Provisioning-deadline expiry bounds its lock wait (fix(server): bound the provisioning-deadline scan's lock wait #4547)
• A dangling provider reference no longer stops the gateway from starting (fix(server): report dangling provider references instead of failing startup #4657)
• Provider refreshes and SSH identity holders can't exhaust a small PostgreSQL data pool (fix(server): scope the SSH identity lock per sandbox and bound data-pool lock holders #4686)
• The SSH host identity lock is scoped per sandbox (fix(server): scope the SSH identity lock per sandbox and bound data-pool lock holders #4686)
• Attach, Detach and UpdateConfig policy updates validate the sandbox as read under its guard
• DeleteWorkspace and sandbox create can't interleave
• A settings write that races a cross-replica delete can't leak into a later same-name sandbox
• Get, List and Watch hide endpoint evidence from a dead supervisor session
• Sandbox create runs middleware and driver validation outside its mutation guards
• Guarded writes can't commit after their lock is released
• feat(kubernetes): add template-driven warm pooling #3508 and feat(sandbox): add delegated identity for token exchange #2772 pick a scope per path when they rebase off the fleet-wide guard (done in those PRs)