feat: granular subsystem commands with parallel scanner execution

Split monolithic scan_updates into individual subsystems (updates/storage/system/docker).
Scanners now run in parallel via goroutines - cuts scan time roughly in half, maybe more.

Agent changes:
- Orchestrator pattern for scanner management
- New scanners: storage (disk metrics), system (cpu/mem/processes)
- New commands: scan_storage, scan_system, scan_docker
- Wrapped existing scanners (APT/DNF/Docker/Windows/Winget) with common interface
- Version bump to 0.1.20

Server changes:
- Migration 015: agent_subsystems table with trigger for auto-init
- Subsystem CRUD: enable/disable, interval (5min-24hr), auto-run toggle
- API routes: /api/v1/agents/:id/subsystems/* (9 endpoints)
- Stats tracking per subsystem

Web UI changes:
- ChatTimeline shows subsystem-specific labels and icons
- AgentScanners got interactive toggles, interval dropdowns, manual trigger buttons
- TypeScript types added for subsystems

Backward compatible with legacy scan_updates - for now. Bugs probably exist somewhere.

aggregator-server/internal/database/queries/agents.go

@@ -67,6 +67,20 @@ func (q *AgentQueries) UpdateAgent(agent *models.Agent) error {
 	return err
 }
 
+// UpdateAgentMetadata updates only the metadata, last_seen, and status fields
+// Used for metrics updates to avoid overwriting version tracking
+func (q *AgentQueries) UpdateAgentMetadata(id uuid.UUID, metadata models.JSONB, status string, lastSeen time.Time) error {
+	query := `
+		UPDATE agents SET
+			last_seen = $1,
+			status = $2,
+			metadata = $3
+		WHERE id = $4
+	`
+	_, err := q.db.Exec(query, lastSeen, status, metadata, id)
+	return err
+}
+
 // ListAgents returns all agents with optional filtering
 func (q *AgentQueries) ListAgents(status, osType string) ([]models.Agent, error) {
 	var agents []models.Agent