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/timeoff/handler.go

@@ -2,12 +2,13 @@ package timeoff
 
 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
 	}
-	requests, err := h.store.List(volunteerID)
+	requests, err := h.store.List(r.Context(), volunteerID)
 	if err != nil {
 		respond.Error(w, http.StatusInternalServerError, "could not list time off requests")
 		return
@@ -48,7 +49,7 @@ func (h *Handler) Create(w http.ResponseWriter, r *http.Request) {
 		respond.Error(w, http.StatusBadRequest, "starts_at and ends_at are required")
 		return
 	}
-	req, err := h.store.Create(claims.VolunteerID, in)
+	req, err := h.store.Create(r.Context(), claims.VolunteerID, in)
 	if err != nil {
 		respond.Error(w, http.StatusInternalServerError, "could not create time off request")
 		return
@@ -73,14 +74,14 @@ func (h *Handler) Review(w http.ResponseWriter, r *http.Request) {
 		respond.Error(w, http.StatusBadRequest, "status must be 'approved' or 'rejected'")
 		return
 	}
-	req, err := h.store.Review(id, claims.VolunteerID, in.Status)
+	req, err := h.store.Review(r.Context(), id, claims.VolunteerID, in.Status)
+	if errors.Is(err, ErrNotFound) {
+		respond.Error(w, http.StatusNotFound, "time off request not found")
+		return
+	}
 	if err != nil {
 		respond.Error(w, http.StatusInternalServerError, "could not review time off request")
 		return
 	}
-	if req == nil {
-		respond.Error(w, http.StatusNotFound, "time off request not found")
-		return
-	}
 	respond.JSON(w, http.StatusOK, req)
 }