Add audit log infrastructure and GET /v1/audit-logs endpoint

Introduces an append-only audit trail for all user and system actions:
sandbox lifecycle (create/pause/resume/destroy/auto-pause), snapshots,
team rename, API key create/revoke, member add/remove/leave/role_update,
and BYOC host add/delete/marked_down/marked_up.

- New audit_logs table (migration) with team_id, actor, resource,
  action, scope (team|admin), status (success|info|warning|error),
  metadata, and created_at
- AuditLogger (internal/audit) with named fire-and-forget methods per
  event; system actor used for background events (HostMonitor, TTL reaper)
- GET /v1/audit-logs: JWT-only, cursor pagination (max 200), multi-value
  filters for resource_type and action (comma-sep or repeated params);
  members see team-scoped events only, admins/owners see all
- AuthContext extended with APIKeyID + APIKeyName so API key requests
  record meaningful actor identity
- HostMonitor wired with AuditLogger for auto-pause and host marked_down

internal/auth/context.go

@@ -8,12 +8,14 @@ const authCtxKey contextKey = 0
 
 // AuthContext is stamped into request context by auth middleware.
 type AuthContext struct {
-	TeamID  string
-	UserID  string // empty when authenticated via API key
-	Email   string // empty when authenticated via API key
-	Name    string // empty when authenticated via API key
-	Role    string // owner, admin, or member; empty when authenticated via API key
-	IsAdmin bool   // platform-level admin; always false when authenticated via API key
+	TeamID     string
+	UserID     string // empty when authenticated via API key
+	Email      string // empty when authenticated via API key
+	Name       string // empty when authenticated via API key
+	Role       string // owner, admin, or member; empty when authenticated via API key
+	IsAdmin    bool   // platform-level admin; always false when authenticated via API key
+	APIKeyID   string // populated when authenticated via API key; empty for JWT auth
+	APIKeyName string // display name of the key, snapshotted at auth time; empty for JWT auth
 }
 
 // WithAuthContext returns a new context with the given AuthContext.