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/models/models.go

@@ -52,11 +52,20 @@ type User struct {
 
 type CheckRecord struct {
 	SiteID    int
+	NodeID    string
 	LatencyNs int64
 	IsUp      bool
 	CheckedAt time.Time
 }
 
+type ProbeNode struct {
+	ID       string
+	Name     string
+	Region   string
+	LastSeen time.Time
+	Version  string
+}
+
 type Backup struct {
 	Sites  []Site        `json:"sites"`
 	Alerts []AlertConfig `json:"alerts"`