Scaling Phase 1: multi-tenant onboarding + replica-safe coordination

Decouples "many users" + "many server replicas" from "many machines" so the
platform is tenant-isolated and horizontally safe on the current single node.

- Per-signup workspaces (cm-auth): a new hosted-identity sign-in provisions and
  owns its own workspace instead of joining the first. Config-gated by
  auth.per_signup_workspace (default off); concurrent first-logins serialized by
  a per-subject advisory lock so no duplicate workspaces.
- Terminal tickets in Postgres (migration 0016, hashed, single-use): any replica
  can redeem a ticket minted by another. Drops the in-process ticket map.
- Container registry in Postgres (migration 0017, agent_containers): Terminal
  and Sandbox managers resolve an agent's container through a shared registry,
  so a 2nd replica reuses it instead of spawning a duplicate. node_id recorded
  as 'local' (Phase 2 hook). Boot reconcile removes only true orphans, so
  terminals now survive a redeploy (tmux sessions resume).
- Per-workspace quotas (cm-api/quota.rs): plan-tier caps on agents + live
  containers, enforced at agent create + terminal spin-up (reconnects allowed),
  returned as HTTP 402. New GET /api/quota surfaces usage vs limits.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

crates/cm-db/src/repo/agents.rs

@@ -6,6 +6,17 @@ use uuid::Uuid;
 
 use crate::DbError;
 
+/// Count of non-deleted agents in a workspace (for per-workspace quotas).
+pub async fn count_active(pool: &PgPool, workspace_id: WorkspaceId) -> Result<i64, DbError> {
+    let n: i64 = sqlx::query_scalar(
+        "SELECT count(*) FROM agents WHERE workspace_id = $1 AND deleted_at IS NULL",
+    )
+    .bind(workspace_id.as_uuid())
+    .fetch_one(pool)
+    .await?;
+    Ok(n)
+}
+
 /// Inserts an agent together with its access policy in one transaction —
 /// an agent without a policy must never be observable (§7.7).
 pub async fn insert(pool: &PgPool, agent: &Agent, policy: &AccessPolicy) -> Result<(), DbError> {