refactor: decompose monitor.go and sqlstore.go into focused files

Split monitor.go (1131→350 lines) into alerts.go, checks.go,
sites.go, and maintenance.go by concern. Split sqlstore.go
(833→470 lines) into sqlstore_alerts.go, sqlstore_history.go,
and sqlstore_maintenance.go by domain. Tests move with their
implementation. Pure reorganization — no behavioral changes.

internal/store/sqlstore_alerts.go

@@ -0,0 +1,126 @@
+package store
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	"gitea.lerkolabs.com/lerkolabs/uptop/internal/models"
+)
+
+func (s *SQLStore) unmarshalSettings(raw string) (map[string]string, error) {
+	decrypted, err := s.decryptSettings(raw)
+	if err != nil {
+		return nil, fmt.Errorf("decrypt settings: %w", err)
+	}
+	var m map[string]string
+	if err := json.Unmarshal([]byte(decrypted), &m); err != nil {
+		return nil, fmt.Errorf("unmarshal settings: %w", err)
+	}
+	return m, nil
+}
+
+func (s *SQLStore) marshalSettings(settings map[string]string) (string, error) {
+	jsonBytes, err := json.Marshal(settings)
+	if err != nil {
+		return "", err
+	}
+	return s.encryptSettings(string(jsonBytes))
+}
+
+func (s *SQLStore) GetAlertByName(ctx context.Context, name string) (models.AlertConfig, error) {
+	var a models.AlertConfig
+	var settingsRaw string
+	err := s.db.QueryRowContext(ctx, s.q("SELECT id, name, type, settings FROM alerts WHERE name = ?"), name).Scan(&a.ID, &a.Name, &a.Type, &settingsRaw)
+	if err != nil {
+		return a, err
+	}
+	a.Settings, err = s.unmarshalSettings(settingsRaw)
+	if err != nil {
+		return a, fmt.Errorf("alert %q: %w", name, err)
+	}
+	return a, nil
+}
+
+func (s *SQLStore) AddAlertReturningID(ctx context.Context, name, aType string, settings map[string]string) (int, error) {
+	stored, err := s.marshalSettings(settings)
+	if err != nil {
+		return 0, err
+	}
+	if s.dollar {
+		var id int
+		err := s.db.QueryRowContext(ctx, s.q("INSERT INTO alerts (name, type, settings) VALUES (?, ?, ?) RETURNING id"), name, aType, stored).Scan(&id)
+		return id, err
+	}
+	result, err := s.db.ExecContext(ctx, s.q("INSERT INTO alerts (name, type, settings) VALUES (?, ?, ?)"), name, aType, stored)
+	if err != nil {
+		return 0, err
+	}
+	id, err := result.LastInsertId()
+	return int(id), err
+}
+
+func (s *SQLStore) GetAllAlerts(ctx context.Context) ([]models.AlertConfig, error) {
+	rows, err := s.db.QueryContext(ctx, "SELECT id, name, type, settings FROM alerts")
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var alerts []models.AlertConfig
+	for rows.Next() {
+		var a models.AlertConfig
+		var settingsRaw string
+		if err := rows.Scan(&a.ID, &a.Name, &a.Type, &settingsRaw); err != nil {
+			return alerts, err
+		}
+		a.Settings, err = s.unmarshalSettings(settingsRaw)
+		if err != nil {
+			return alerts, fmt.Errorf("alert %q: %w", a.Name, err)
+		}
+		alerts = append(alerts, a)
+	}
+	return alerts, rows.Err()
+}
+
+func (s *SQLStore) GetAlert(ctx context.Context, id int) (models.AlertConfig, error) {
+	var a models.AlertConfig
+	var settingsRaw string
+	err := s.db.QueryRowContext(ctx, s.q("SELECT id, name, type, settings FROM alerts WHERE id = ?"), id).Scan(&a.ID, &a.Name, &a.Type, &settingsRaw)
+	if err != nil {
+		return a, err
+	}
+	a.Settings, err = s.unmarshalSettings(settingsRaw)
+	if err != nil {
+		return a, fmt.Errorf("alert %d: %w", id, err)
+	}
+	return a, nil
+}
+
+func (s *SQLStore) AddAlert(ctx context.Context, name, aType string, settings map[string]string) error {
+	stored, err := s.marshalSettings(settings)
+	if err != nil {
+		return err
+	}
+	_, err = s.db.ExecContext(ctx, s.q("INSERT INTO alerts (name, type, settings) VALUES (?, ?, ?)"), name, aType, stored)
+	return err
+}
+
+func (s *SQLStore) UpdateAlert(ctx context.Context, id int, name, aType string, settings map[string]string) error {
+	stored, err := s.marshalSettings(settings)
+	if err != nil {
+		return err
+	}
+	_, err = s.db.ExecContext(ctx, s.q("UPDATE alerts SET name=?, type=?, settings=? WHERE id=?"), name, aType, stored, id)
+	return err
+}
+
+func (s *SQLStore) DeleteAlert(ctx context.Context, id int) error {
+	if _, err := s.db.ExecContext(ctx, s.q("UPDATE sites SET alert_id = 0 WHERE alert_id = ?"), id); err != nil {
+		return err
+	}
+	if _, err := s.db.ExecContext(ctx, s.q("DELETE FROM alerts WHERE id=?"), id); err != nil {
+		return err
+	}
+	s.dialect.ResetSequenceOnEmpty(s.db, "alerts")
+	return nil
+}