Conform Go code to project conventions

- Propagate context.Context through all exported store/service methods
  that perform I/O; use QueryContext/ExecContext/QueryRowContext throughout
- Add package-level sentinel errors (ErrNotFound, ErrAlreadyCheckedIn,
  ErrNotCheckedIn) and replace nil,nil returns with explicit errors
- Update handlers to use errors.Is() instead of nil checks, with correct
  HTTP status codes per error type
- Fix SQLite datetime('now') → MySQL NOW() in volunteer, schedule,
  timeoff, and checkin stores
- Refactor db.Migrate to execute schema statements individually (MySQL
  driver does not support multi-statement Exec)
- Fix import grouping in handler files (stdlib, external, internal)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/notification/handler.go

@@ -1,12 +1,13 @@
 package notification
 
 import (
+	"errors"
 	"net/http"
 	"strconv"
 
-	"github.com/go-chi/chi/v5"
 	"git.unsupervised.ca/walkies/internal/respond"
 	"git.unsupervised.ca/walkies/internal/server/middleware"
+	"github.com/go-chi/chi/v5"
 )
 
 type Handler struct {
@@ -20,7 +21,7 @@ func NewHandler(store *Store) *Handler {
 // GET /api/v1/notifications
 func (h *Handler) List(w http.ResponseWriter, r *http.Request) {
 	claims := middleware.ClaimsFromContext(r.Context())
-	notifications, err := h.store.ListForVolunteer(claims.VolunteerID)
+	notifications, err := h.store.ListForVolunteer(r.Context(), claims.VolunteerID)
 	if err != nil {
 		respond.Error(w, http.StatusInternalServerError, "could not list notifications")
 		return
@@ -39,14 +40,14 @@ func (h *Handler) MarkRead(w http.ResponseWriter, r *http.Request) {
 		respond.Error(w, http.StatusBadRequest, "invalid id")
 		return
 	}
-	n, err := h.store.MarkRead(id, claims.VolunteerID)
+	n, err := h.store.MarkRead(r.Context(), id, claims.VolunteerID)
+	if errors.Is(err, ErrNotFound) {
+		respond.Error(w, http.StatusNotFound, "notification not found")
+		return
+	}
 	if err != nil {
 		respond.Error(w, http.StatusInternalServerError, "could not mark notification as read")
 		return
 	}
-	if n == nil {
-		respond.Error(w, http.StatusNotFound, "notification not found")
-		return
-	}
 	respond.JSON(w, http.StatusOK, n)
 }

internal/notification/notification.go

@@ -1,12 +1,15 @@
 package notification
 
 import (
+	"context"
 	"database/sql"
 	"errors"
 	"fmt"
 	"time"
 )
 
+var ErrNotFound = fmt.Errorf("notification not found")
+
 type Notification struct {
 	ID          int64     `json:"id"`
 	VolunteerID int64     `json:"volunteer_id"`
@@ -23,8 +26,8 @@ func NewStore(db *sql.DB) *Store {
 	return &Store{db: db}
 }
 
-func (s *Store) Create(volunteerID int64, message string) (*Notification, error) {
-	res, err := s.db.Exec(
+func (s *Store) Create(ctx context.Context, volunteerID int64, message string) (*Notification, error) {
+	res, err := s.db.ExecContext(ctx,
 		`INSERT INTO notifications (volunteer_id, message) VALUES (?, ?)`,
 		volunteerID, message,
 	)
@@ -32,17 +35,17 @@ func (s *Store) Create(volunteerID int64, message string) (*Notification, error)
 		return nil, fmt.Errorf("insert notification: %w", err)
 	}
 	id, _ := res.LastInsertId()
-	return s.GetByID(id)
+	return s.GetByID(ctx, id)
 }
 
-func (s *Store) GetByID(id int64) (*Notification, error) {
+func (s *Store) GetByID(ctx context.Context, id int64) (*Notification, error) {
 	n := &Notification{}
 	var createdAt string
-	err := s.db.QueryRow(
+	err := s.db.QueryRowContext(ctx,
 		`SELECT id, volunteer_id, message, read, created_at FROM notifications WHERE id = ?`, id,
 	).Scan(&n.ID, &n.VolunteerID, &n.Message, &n.Read, &createdAt)
 	if errors.Is(err, sql.ErrNoRows) {
-		return nil, nil
+		return nil, ErrNotFound
 	}
 	if err != nil {
 		return nil, fmt.Errorf("get notification: %w", err)
@@ -51,8 +54,8 @@ func (s *Store) GetByID(id int64) (*Notification, error) {
 	return n, nil
 }
 
-func (s *Store) ListForVolunteer(volunteerID int64) ([]Notification, error) {
-	rows, err := s.db.Query(
+func (s *Store) ListForVolunteer(ctx context.Context, volunteerID int64) ([]Notification, error) {
+	rows, err := s.db.QueryContext(ctx,
 		`SELECT id, volunteer_id, message, read, created_at FROM notifications WHERE volunteer_id = ? ORDER BY created_at DESC`,
 		volunteerID,
 	)
@@ -74,17 +77,17 @@ func (s *Store) ListForVolunteer(volunteerID int64) ([]Notification, error) {
 	return notifications, rows.Err()
 }
 
-func (s *Store) MarkRead(id, volunteerID int64) (*Notification, error) {
-	result, err := s.db.Exec(
+func (s *Store) MarkRead(ctx context.Context, id, volunteerID int64) (*Notification, error) {
+	result, err := s.db.ExecContext(ctx,
 		`UPDATE notifications SET read = 1 WHERE id = ? AND volunteer_id = ?`,
 		id, volunteerID,
 	)
 	if err != nil {
 		return nil, fmt.Errorf("mark read: %w", err)
 	}
-	rows, _ := result.RowsAffected()
-	if rows == 0 {
-		return nil, nil
+	affected, _ := result.RowsAffected()
+	if affected == 0 {
+		return nil, ErrNotFound
 	}
-	return s.GetByID(id)
+	return s.GetByID(ctx, id)
 }