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.

internal/store/postgres.go

@@ -44,6 +44,13 @@ func (d *PostgresDialect) CreateTablesSQL() []string {
 			is_up BOOLEAN, checked_at TIMESTAMP DEFAULT NOW()
 		)`,
 		`CREATE INDEX IF NOT EXISTS idx_check_history_site ON check_history(site_id, checked_at DESC)`,
+		`CREATE TABLE IF NOT EXISTS nodes (
+			id TEXT PRIMARY KEY,
+			name TEXT NOT NULL,
+			region TEXT DEFAULT '',
+			last_seen TIMESTAMP DEFAULT NOW(),
+			version TEXT DEFAULT ''
+		)`,
 	}
 }
 
@@ -60,9 +67,14 @@ func (d *PostgresDialect) MigrationsSQL() []string {
 		"ALTER TABLE sites ADD COLUMN IF NOT EXISTS dns_server TEXT DEFAULT ''",
 		"ALTER TABLE sites ADD COLUMN IF NOT EXISTS ignore_tls BOOLEAN DEFAULT FALSE",
 		"ALTER TABLE sites ADD COLUMN IF NOT EXISTS paused BOOLEAN DEFAULT FALSE",
+		"ALTER TABLE check_history ADD COLUMN IF NOT EXISTS node_id TEXT DEFAULT ''",
 	}
 }
 
+func (d *PostgresDialect) UpsertNodeSQL() string {
+	return "INSERT INTO nodes (id, name, region, last_seen, version) VALUES ($1, $2, $3, NOW(), $4) ON CONFLICT (id) DO UPDATE SET name = EXCLUDED.name, region = EXCLUDED.region, last_seen = NOW(), version = EXCLUDED.version"
+}
+
 func (d *PostgresDialect) ResetSequenceOnEmpty(db *sql.DB, table string) {}
 
 func (d *PostgresDialect) ImportWipe(tx *sql.Tx) {

internal/store/sqlite.go

@@ -44,6 +44,13 @@ func (d *SQLiteDialect) CreateTablesSQL() []string {
 			is_up BOOLEAN, checked_at DATETIME DEFAULT CURRENT_TIMESTAMP
 		)`,
 		`CREATE INDEX IF NOT EXISTS idx_check_history_site ON check_history(site_id, checked_at DESC)`,
+		`CREATE TABLE IF NOT EXISTS nodes (
+			id TEXT PRIMARY KEY,
+			name TEXT NOT NULL,
+			region TEXT DEFAULT '',
+			last_seen DATETIME DEFAULT CURRENT_TIMESTAMP,
+			version TEXT DEFAULT ''
+		)`,
 	}
 }
 
@@ -60,9 +67,14 @@ func (d *SQLiteDialect) MigrationsSQL() []string {
 		"ALTER TABLE sites ADD COLUMN dns_server TEXT DEFAULT ''",
 		"ALTER TABLE sites ADD COLUMN ignore_tls BOOLEAN DEFAULT 0",
 		"ALTER TABLE sites ADD COLUMN paused BOOLEAN DEFAULT 0",
+		"ALTER TABLE check_history ADD COLUMN node_id TEXT DEFAULT ''",
 	}
 }
 
+func (d *SQLiteDialect) UpsertNodeSQL() string {
+	return "INSERT OR REPLACE INTO nodes (id, name, region, last_seen, version) VALUES (?, ?, ?, CURRENT_TIMESTAMP, ?)"
+}
+
 func (d *SQLiteDialect) ResetSequenceOnEmpty(db *sql.DB, table string) {
 	var count int
 	db.QueryRow("SELECT COUNT(*) FROM " + table).Scan(&count)

internal/store/sqlstore.go

@@ -247,7 +247,11 @@ func (s *SQLStore) DeleteUser(id int) error {
 }
 
 func (s *SQLStore) SaveCheck(siteID int, latencyNs int64, isUp bool) error {
-	_, err := s.db.Exec(s.q("INSERT INTO check_history (site_id, latency_ns, is_up) VALUES (?, ?, ?)"), siteID, latencyNs, isUp)
+	return s.SaveCheckFromNode(siteID, "", latencyNs, isUp)
+}
+
+func (s *SQLStore) SaveCheckFromNode(siteID int, nodeID string, latencyNs int64, isUp bool) error {
+	_, err := s.db.Exec(s.q("INSERT INTO check_history (site_id, node_id, latency_ns, is_up) VALUES (?, ?, ?, ?)"), siteID, nodeID, latencyNs, isUp)
 	if err != nil {
 		return err
 	}
@@ -257,6 +261,45 @@ func (s *SQLStore) SaveCheck(siteID int, latencyNs int64, isUp bool) error {
 	return err
 }
 
+func (s *SQLStore) RegisterNode(node models.ProbeNode) error {
+	_, err := s.db.Exec(s.dialect.UpsertNodeSQL(), node.ID, node.Name, node.Region, node.Version)
+	return err
+}
+
+func (s *SQLStore) GetNode(id string) (models.ProbeNode, error) {
+	var n models.ProbeNode
+	err := s.db.QueryRow(s.q("SELECT id, name, region, last_seen, version FROM nodes WHERE id = ?"), id).
+		Scan(&n.ID, &n.Name, &n.Region, &n.LastSeen, &n.Version)
+	return n, err
+}
+
+func (s *SQLStore) GetAllNodes() ([]models.ProbeNode, error) {
+	rows, err := s.db.Query("SELECT id, name, region, last_seen, version FROM nodes ORDER BY region, name")
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var nodes []models.ProbeNode
+	for rows.Next() {
+		var n models.ProbeNode
+		if err := rows.Scan(&n.ID, &n.Name, &n.Region, &n.LastSeen, &n.Version); err != nil {
+			return nodes, err
+		}
+		nodes = append(nodes, n)
+	}
+	return nodes, rows.Err()
+}
+
+func (s *SQLStore) UpdateNodeLastSeen(id string) error {
+	_, err := s.db.Exec(s.q("UPDATE nodes SET last_seen = CURRENT_TIMESTAMP WHERE id = ?"), id)
+	return err
+}
+
+func (s *SQLStore) DeleteNode(id string) error {
+	_, err := s.db.Exec(s.q("DELETE FROM nodes WHERE id = ?"), id)
+	return err
+}
+
 func (s *SQLStore) LoadAllHistory(limit int) (map[int][]models.CheckRecord, error) {
 	result := make(map[int][]models.CheckRecord)
 	rows, err := s.db.Query(s.q(`

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