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/volunteer/handler.go

@@ -2,12 +2,13 @@ package volunteer
 
 import (
 	"encoding/json"
+	"errors"
 	"net/http"
 	"strconv"
 
-	"github.com/go-chi/chi/v5"
 	"git.unsupervised.ca/walkies/internal/auth"
 	"git.unsupervised.ca/walkies/internal/respond"
+	"github.com/go-chi/chi/v5"
 )
 
 type Handler struct {
@@ -38,7 +39,7 @@ func (h *Handler) Register(w http.ResponseWriter, r *http.Request) {
 		respond.Error(w, http.StatusInternalServerError, "could not hash password")
 		return
 	}
-	v, err := h.store.Create(in.Name, in.Email, hash, in.Role)
+	v, err := h.store.Create(r.Context(), in.Name, in.Email, hash, in.Role)
 	if err != nil {
 		respond.Error(w, http.StatusConflict, "email already in use")
 		return
@@ -56,7 +57,7 @@ func (h *Handler) Login(w http.ResponseWriter, r *http.Request) {
 		respond.Error(w, http.StatusBadRequest, "invalid request body")
 		return
 	}
-	token, err := h.authSvc.Login(body.Email, body.Password)
+	token, err := h.authSvc.Login(r.Context(), body.Email, body.Password)
 	if err != nil {
 		respond.Error(w, http.StatusUnauthorized, "invalid credentials")
 		return
@@ -66,7 +67,7 @@ func (h *Handler) Login(w http.ResponseWriter, r *http.Request) {
 
 // GET /api/v1/volunteers
 func (h *Handler) List(w http.ResponseWriter, r *http.Request) {
-	volunteers, err := h.store.List(true)
+	volunteers, err := h.store.List(r.Context(), true)
 	if err != nil {
 		respond.Error(w, http.StatusInternalServerError, "could not list volunteers")
 		return
@@ -84,13 +85,13 @@ func (h *Handler) Get(w http.ResponseWriter, r *http.Request) {
 		respond.Error(w, http.StatusBadRequest, "invalid id")
 		return
 	}
-	v, err := h.store.GetByID(id)
-	if err != nil {
-		respond.Error(w, http.StatusInternalServerError, "could not get volunteer")
+	v, err := h.store.GetByID(r.Context(), id)
+	if errors.Is(err, ErrNotFound) {
+		respond.Error(w, http.StatusNotFound, "volunteer not found")
 		return
 	}
-	if v == nil {
-		respond.Error(w, http.StatusNotFound, "volunteer not found")
+	if err != nil {
+		respond.Error(w, http.StatusInternalServerError, "could not get volunteer")
 		return
 	}
 	respond.JSON(w, http.StatusOK, v)
@@ -108,14 +109,14 @@ func (h *Handler) Update(w http.ResponseWriter, r *http.Request) {
 		respond.Error(w, http.StatusBadRequest, "invalid request body")
 		return
 	}
-	v, err := h.store.Update(id, in)
+	v, err := h.store.Update(r.Context(), id, in)
+	if errors.Is(err, ErrNotFound) {
+		respond.Error(w, http.StatusNotFound, "volunteer not found")
+		return
+	}
 	if err != nil {
 		respond.Error(w, http.StatusInternalServerError, "could not update volunteer")
 		return
 	}
-	if v == nil {
-		respond.Error(w, http.StatusNotFound, "volunteer not found")
-		return
-	}
 	respond.JSON(w, http.StatusOK, v)
 }