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/checkin/handler.go

@@ -2,6 +2,7 @@ package checkin
 
 import (
 	"encoding/json"
+	"errors"
 	"net/http"
 
 	"git.unsupervised.ca/walkies/internal/respond"
@@ -24,11 +25,15 @@ func (h *Handler) CheckIn(w http.ResponseWriter, r *http.Request) {
 		respond.Error(w, http.StatusBadRequest, "invalid request body")
 		return
 	}
-	ci, err := h.store.CheckIn(claims.VolunteerID, in)
-	if err != nil {
+	ci, err := h.store.CheckIn(r.Context(), claims.VolunteerID, in)
+	if errors.Is(err, ErrAlreadyCheckedIn) {
 		respond.Error(w, http.StatusConflict, err.Error())
 		return
 	}
+	if err != nil {
+		respond.Error(w, http.StatusInternalServerError, "could not check in")
+		return
+	}
 	respond.JSON(w, http.StatusCreated, ci)
 }
 
@@ -37,11 +42,15 @@ func (h *Handler) CheckOut(w http.ResponseWriter, r *http.Request) {
 	claims := middleware.ClaimsFromContext(r.Context())
 	var in CheckOutInput
 	json.NewDecoder(r.Body).Decode(&in)
-	ci, err := h.store.CheckOut(claims.VolunteerID, in)
-	if err != nil {
+	ci, err := h.store.CheckOut(r.Context(), claims.VolunteerID, in)
+	if errors.Is(err, ErrNotCheckedIn) {
 		respond.Error(w, http.StatusConflict, err.Error())
 		return
 	}
+	if err != nil {
+		respond.Error(w, http.StatusInternalServerError, "could not check out")
+		return
+	}
 	respond.JSON(w, http.StatusOK, ci)
 }
 
@@ -52,7 +61,7 @@ func (h *Handler) History(w http.ResponseWriter, r *http.Request) {
 	if claims.Role != "admin" {
 		volunteerID = claims.VolunteerID
 	}
-	history, err := h.store.History(volunteerID)
+	history, err := h.store.History(r.Context(), volunteerID)
 	if err != nil {
 		respond.Error(w, http.StatusInternalServerError, "could not get history")
 		return