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/schedule/handler.go

@@ -2,12 +2,13 @@ package schedule
 
 import (
 	"encoding/json"
+	"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 {
@@ -25,7 +26,7 @@ func (h *Handler) List(w http.ResponseWriter, r *http.Request) {
 	if claims.Role != "admin" {
 		volunteerID = claims.VolunteerID
 	}
-	schedules, err := h.store.List(volunteerID)
+	schedules, err := h.store.List(r.Context(), volunteerID)
 	if err != nil {
 		respond.Error(w, http.StatusInternalServerError, "could not list schedules")
 		return
@@ -51,7 +52,7 @@ func (h *Handler) Create(w http.ResponseWriter, r *http.Request) {
 		respond.Error(w, http.StatusBadRequest, "title, starts_at, and ends_at are required")
 		return
 	}
-	sc, err := h.store.Create(in)
+	sc, err := h.store.Create(r.Context(), in)
 	if err != nil {
 		respond.Error(w, http.StatusInternalServerError, "could not create schedule")
 		return
@@ -71,13 +72,13 @@ func (h *Handler) Update(w http.ResponseWriter, r *http.Request) {
 		respond.Error(w, http.StatusBadRequest, "invalid request body")
 		return
 	}
-	sc, err := h.store.Update(id, in)
-	if err != nil {
-		respond.Error(w, http.StatusInternalServerError, "could not update schedule")
+	sc, err := h.store.Update(r.Context(), id, in)
+	if errors.Is(err, ErrNotFound) {
+		respond.Error(w, http.StatusNotFound, "schedule not found")
 		return
 	}
-	if sc == nil {
-		respond.Error(w, http.StatusNotFound, "schedule not found")
+	if err != nil {
+		respond.Error(w, http.StatusInternalServerError, "could not update schedule")
 		return
 	}
 	respond.JSON(w, http.StatusOK, sc)
@@ -90,7 +91,7 @@ func (h *Handler) Delete(w http.ResponseWriter, r *http.Request) {
 		respond.Error(w, http.StatusBadRequest, "invalid id")
 		return
 	}
-	if err := h.store.Delete(id); err != nil {
+	if err := h.store.Delete(r.Context(), id); err != nil {
 		respond.Error(w, http.StatusInternalServerError, "could not delete schedule")
 		return
 	}