feat: add runtime storage cleanup with per-category retention (#310) (#313)

Runtime artifacts (agent workspaces, tool-output spill, C2 payloads, chat
uploads, workflow checkpoints, diagnostic logs) previously accumulated
without bound: most were only removed when a conversation or project was
deleted, and tmp/c2 plus workflow checkpoints were never removed at all.

Add a storage cleaner with named per-category tasks (Gitea-style), a
settings page tab, and a background sweep that is off by default so
upgrading never deletes existing data.

Safety properties, since mis-deleting live task data costs far more than
the disk saved:
- dry-run is the default; a real cleanup requires dry_run=false together
  with confirm=true at the API layer, not just a frontend dialog
- sessions active within active_grace_hours are always skipped, and a
  failed activity lookup skips conservatively (fail closed)
- directories whose conversation/project no longer exists are reclaimed
  as orphans after orphan_grace_days
- scanners never follow symlinks and every candidate path is confined to
  its category root; deletion renames to a .tmp-for-deletion marker first
  so a crash leaves recoverable residue instead of a half-deleted dir
- storage:* permissions are admin-only; without the grantSystemRolePermissions
  skip the default branch would have given operators an irreversible
  file-deletion right

Also fix two confirmed leaks: DeleteConversation left chat_uploads files
on disk (their rows already vanished via ON DELETE CASCADE), and workflow
checkpoints had no deletion path at all.

Co-authored-by: Parallels <parallels@kali-linux-2025-2.localdomain>

internal/database/rbac.go

@@ -371,7 +371,7 @@ func grantSystemRolePermissions(tx *sql.Tx, permissions map[string]string) error
 			if _, err := tx.Exec(`INSERT OR IGNORE INTO rbac_role_permissions (role_id, permission_key, created_at) VALUES (?, ?, ?)`, RBACSystemRoleAuditor, key, now); err != nil {
 				return err
 			}
-		case strings.HasPrefix(key, "rbac:"), strings.HasPrefix(key, "config:"), strings.HasPrefix(key, "terminal:"), strings.HasPrefix(key, "audit:"):
+		case strings.HasPrefix(key, "rbac:"), strings.HasPrefix(key, "config:"), strings.HasPrefix(key, "terminal:"), strings.HasPrefix(key, "audit:"), strings.HasPrefix(key, "storage:"):
 			continue
 		case key == "mcp:write" || key == "mcp:external:execute":
 			continue