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/dialect.go

@@ -10,6 +10,7 @@ type Dialect interface {
 	ResetSequenceOnEmpty(db *sql.DB, table string)
 	ImportWipe(tx *sql.Tx)
 	ImportResetSequences(tx *sql.Tx)
+	UpsertNodeSQL() string
 }
 
 // rewritePlaceholders converts ? markers to $1, $2, etc. for Postgres.