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/db/db.go

@@ -1,6 +1,7 @@
 package db
 
 import (
+	"context"
 	"database/sql"
 	"fmt"
 
@@ -18,7 +19,11 @@ func Open(dsn string) (*sql.DB, error) {
 	return db, nil
 }
 
-func Migrate(db *sql.DB) error {
-	_, err := db.Exec(schema)
-	return err
+func Migrate(ctx context.Context, db *sql.DB) error {
+	for _, stmt := range statements {
+		if _, err := db.ExecContext(ctx, stmt); err != nil {
+			return fmt.Errorf("migrate: %w", err)
+		}
+	}
+	return nil
 }