fix: Remove duplicate scan logging to prevent storage/system scans on Updates page

BREAKING CHANGE: Storage and system scans no longer create entries in update_logs

**Problem**
- Storage scans were appearing on Updates page (mixed with package updates)
- System scans were appearing on Updates page (mixed with package updates)
- Duplicate "Scan All" entries from collective + individual logging

**Root Cause**
Scan handlers were calling both ReportLog() and dedicated endpoints:
- reportLogWithAck → POST /api/v1/agents/:id/logs → update_logs table
- This caused storage/system metrics to appear alongside package updates

**Fix**
Removed ALL ReportLog() calls from scan handlers:
1. handleScanUpdatesV2 (lines 44-46): Removed collective logging
2. handleScanStorage (lines 103-105): Use only ReportStorageMetrics
3. handleScanSystem (lines 189-191): Use only ReportMetrics
4. handleScanDocker (lines 269-271): Use only ReportDockerImages

**Verification**
- All 4 handlers have working dedicated endpoints (verified via subagent)
- Routes already registered: POST /storage-metrics, POST /metrics, etc.
- Frontend queries correct endpoints (verified)
- No data loss: dedicated endpoints store in proper tables

**Result**
- Storage scans → storage_metrics table → Storage page only ✅
- System scans → system reporting → System page only ✅
- Package updates → update_logs table → Updates page only ✅
- No duplicate "Scan All" entries ✅

**Files Changed**
- aggregator-agent/cmd/agent/subsystem_handlers.go: Removed 20 lines of ReportLog calls
- internal/api/handlers/agents.go: Command recovery enhancements
- internal/api/handlers/updates.go: Subsystem extraction logic
- internal/database/queries/commands.go: GetStuckCommands query

aggregator-server/internal/models/update.go

@@ -58,6 +58,7 @@ type UpdateLog struct {
 	AgentID         uuid.UUID  `json:"agent_id" db:"agent_id"`
 	UpdatePackageID *uuid.UUID `json:"update_package_id,omitempty" db:"update_package_id"`
 	Action          string     `json:"action" db:"action"`
+	Subsystem       string     `json:"subsystem,omitempty" db:"subsystem"`
 	Result          string     `json:"result" db:"result"`
 	Stdout          string     `json:"stdout" db:"stdout"`
 	Stderr          string     `json:"stderr" db:"stderr"`
@@ -70,6 +71,7 @@ type UpdateLog struct {
 type UpdateLogRequest struct {
 	CommandID       string    `json:"command_id"`
 	Action          string    `json:"action" binding:"required"`
+	Subsystem       string    `json:"subsystem,omitempty"`
 	Result          string    `json:"result" binding:"required"`
 	Stdout          string    `json:"stdout"`
 	Stderr          string    `json:"stderr"`