fix: Address code review issues and fix miner start deadlock

- Remove deprecated GetDB() function that exposed raw DB pointer
- Fix GetLatestHashrate to distinguish sql.ErrNoRows from real errors
- Document async saves in PeerRegistry mutation methods
- Fix deadlock in XMRig/TTMiner Start() by moving CheckInstallation
  call before acquiring the main lock (Go RWMutex doesn't allow
  recursive locking)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pkg/database/database.go

@@ -106,18 +106,6 @@ func IsInitialized() bool {
 	return db != nil
 }
 
-// GetDB returns the database connection (for advanced queries).
-//
-// Deprecated: This function is unsafe for concurrent use because the returned
-// pointer may become invalid if Close() is called by another goroutine after
-// GetDB() returns. Use the dedicated query functions (InsertHashratePoint,
-// GetHashrateHistory, etc.) instead, which handle locking internally.
-func GetDB() *sql.DB {
-	dbMu.RLock()
-	defer dbMu.RUnlock()
-	return db
-}
-
 // createTables creates all required database tables
 func createTables() error {
 	schema := `

pkg/database/hashrate.go

@@ -2,6 +2,7 @@ package database
 
 import (
 	"context"
+	"database/sql"
 	"fmt"
 	"log"
 	"time"
@@ -163,7 +164,10 @@ func GetLatestHashrate(minerName string) (*HashratePoint, error) {
 	`, minerName).Scan(&point.Timestamp, &point.Hashrate)
 
 	if err != nil {
-		return nil, nil // Not found is not an error
+		if err == sql.ErrNoRows {
+			return nil, nil // No data found is not an error
+		}
+		return nil, fmt.Errorf("failed to get latest hashrate: %w", err)
 	}
 
 	return &point, nil

pkg/mining/ttminer_start.go

@@ -13,6 +13,17 @@ import (
 
 // Start launches the TT-Miner with the given configuration.
 func (m *TTMiner) Start(config *Config) error {
+	// Check installation BEFORE acquiring lock (CheckInstallation takes its own locks)
+	m.mu.RLock()
+	needsInstallCheck := m.MinerBinary == ""
+	m.mu.RUnlock()
+
+	if needsInstallCheck {
+		if _, err := m.CheckInstallation(); err != nil {
+			return err // Propagate the detailed error from CheckInstallation
+		}
+	}
+
 	m.mu.Lock()
 	defer m.mu.Unlock()
 
@@ -20,13 +31,6 @@ func (m *TTMiner) Start(config *Config) error {
 		return errors.New("miner is already running")
 	}
 
-	// If the binary path isn't set, run CheckInstallation to find it.
-	if m.MinerBinary == "" {
-		if _, err := m.CheckInstallation(); err != nil {
-			return err // Propagate the detailed error from CheckInstallation
-		}
-	}
-
 	if m.API != nil && config.HTTPPort != 0 {
 		m.API.ListenPort = config.HTTPPort
 	} else if m.API != nil && m.API.ListenPort == 0 {

pkg/mining/xmrig_start.go

@@ -15,6 +15,17 @@ import (
 
 // Start launches the XMRig miner with the specified configuration.
 func (m *XMRigMiner) Start(config *Config) error {
+	// Check installation BEFORE acquiring lock (CheckInstallation takes its own locks)
+	m.mu.RLock()
+	needsInstallCheck := m.MinerBinary == ""
+	m.mu.RUnlock()
+
+	if needsInstallCheck {
+		if _, err := m.CheckInstallation(); err != nil {
+			return err // Propagate the detailed error from CheckInstallation
+		}
+	}
+
 	m.mu.Lock()
 	defer m.mu.Unlock()
 
@@ -22,13 +33,6 @@ func (m *XMRigMiner) Start(config *Config) error {
 		return errors.New("miner is already running")
 	}
 
-	// If the binary path isn't set, run CheckInstallation to find it.
-	if m.MinerBinary == "" {
-		if _, err := m.CheckInstallation(); err != nil {
-			return err // Propagate the detailed error from CheckInstallation
-		}
-	}
-
 	if m.API != nil && config.HTTPPort != 0 {
 		m.API.ListenPort = config.HTTPPort
 	} else if m.API != nil && m.API.ListenPort == 0 {

pkg/node/peer.go

@@ -91,6 +91,8 @@ func NewPeerRegistryWithPath(peersPath string) (*PeerRegistry, error) {
 }
 
 // AddPeer adds a new peer to the registry.
+// Note: Persistence is debounced (writes batched every 5s). Call Close() to ensure
+// all changes are flushed to disk before shutdown.
 func (r *PeerRegistry) AddPeer(peer *Peer) error {
 	r.mu.Lock()
 
@@ -120,6 +122,7 @@ func (r *PeerRegistry) AddPeer(peer *Peer) error {
 }
 
 // UpdatePeer updates an existing peer's information.
+// Note: Persistence is debounced. Call Close() to flush before shutdown.
 func (r *PeerRegistry) UpdatePeer(peer *Peer) error {
 	r.mu.Lock()
 
@@ -136,6 +139,7 @@ func (r *PeerRegistry) UpdatePeer(peer *Peer) error {
 }
 
 // RemovePeer removes a peer from the registry.
+// Note: Persistence is debounced. Call Close() to flush before shutdown.
 func (r *PeerRegistry) RemovePeer(id string) error {
 	r.mu.Lock()
 
@@ -180,6 +184,7 @@ func (r *PeerRegistry) ListPeers() []*Peer {
 }
 
 // UpdateMetrics updates a peer's performance metrics.
+// Note: Persistence is debounced. Call Close() to flush before shutdown.
 func (r *PeerRegistry) UpdateMetrics(id string, pingMS, geoKM float64, hops int) error {
 	r.mu.Lock()
 
@@ -201,6 +206,7 @@ func (r *PeerRegistry) UpdateMetrics(id string, pingMS, geoKM float64, hops int)
 }
 
 // UpdateScore updates a peer's reliability score.
+// Note: Persistence is debounced. Call Close() to flush before shutdown.
 func (r *PeerRegistry) UpdateScore(id string, score float64) error {
 	r.mu.Lock()