feat(cluster): add distributed probing foundation — schema, models, and probe APIs

Add node-aware check history and probe registration infrastructure:
- ProbeNode model and nodes table (SQLite + Postgres)
- node_id column on check_history for multi-source tracking
- Store interface: RegisterNode, GetNode, GetAllNodes, DeleteNode, SaveCheckFromNode
- Dialect: UpsertNodeSQL (INSERT OR REPLACE / ON CONFLICT)
- API endpoints: POST /api/probe/register, GET /api/probe/assignments, POST /api/probe/results
- Backward compatible: existing SaveCheck wraps SaveCheckFromNode with empty node_id

internal/store/store.go

@@ -35,8 +35,16 @@ type Store interface {
 
 	// History
 	SaveCheck(siteID int, latencyNs int64, isUp bool) error
+	SaveCheckFromNode(siteID int, nodeID string, latencyNs int64, isUp bool) error
 	LoadAllHistory(limit int) (map[int][]models.CheckRecord, error)
 
+	// Nodes
+	RegisterNode(node models.ProbeNode) error
+	GetNode(id string) (models.ProbeNode, error)
+	GetAllNodes() ([]models.ProbeNode, error)
+	UpdateNodeLastSeen(id string) error
+	DeleteNode(id string) error
+
 	// Backup & Restore
 	ExportData() (models.Backup, error)
 	ImportData(data models.Backup) error