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
 }

internal/db/schema.go

@@ -1,7 +1,7 @@
 package db
 
-const schema = `
-CREATE TABLE IF NOT EXISTS volunteers (
+var statements = []string{
+	`CREATE TABLE IF NOT EXISTS volunteers (
     id          INT AUTO_INCREMENT PRIMARY KEY,
     name        VARCHAR(255) NOT NULL,
     email       VARCHAR(255) NOT NULL UNIQUE,
@@ -10,9 +10,8 @@ CREATE TABLE IF NOT EXISTS volunteers (
     active      TINYINT NOT NULL DEFAULT 1,
     created_at  TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
     updated_at  TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
-) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
-
-CREATE TABLE IF NOT EXISTS schedules (
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci`,
+	`CREATE TABLE IF NOT EXISTS schedules (
     id            INT AUTO_INCREMENT PRIMARY KEY,
     volunteer_id  INT NOT NULL,
     title         VARCHAR(255) NOT NULL,
@@ -23,9 +22,8 @@ CREATE TABLE IF NOT EXISTS schedules (
     updated_at    TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
     FOREIGN KEY (volunteer_id) REFERENCES volunteers(id) ON DELETE CASCADE,
     INDEX idx_volunteer_id (volunteer_id)
-) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
-
-CREATE TABLE IF NOT EXISTS time_off_requests (
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci`,
+	`CREATE TABLE IF NOT EXISTS time_off_requests (
     id            INT AUTO_INCREMENT PRIMARY KEY,
     volunteer_id  INT NOT NULL,
     starts_at     DATETIME NOT NULL,
@@ -40,9 +38,8 @@ CREATE TABLE IF NOT EXISTS time_off_requests (
     FOREIGN KEY (reviewed_by) REFERENCES volunteers(id) ON DELETE SET NULL,
     INDEX idx_volunteer_id (volunteer_id),
     INDEX idx_status (status)
-) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
-
-CREATE TABLE IF NOT EXISTS checkins (
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci`,
+	`CREATE TABLE IF NOT EXISTS checkins (
     id            INT AUTO_INCREMENT PRIMARY KEY,
     volunteer_id  INT NOT NULL,
     schedule_id   INT,
@@ -53,9 +50,8 @@ CREATE TABLE IF NOT EXISTS checkins (
     FOREIGN KEY (schedule_id) REFERENCES schedules(id) ON DELETE SET NULL,
     INDEX idx_volunteer_id (volunteer_id),
     INDEX idx_schedule_id (schedule_id)
-) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
-
-CREATE TABLE IF NOT EXISTS notifications (
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci`,
+	`CREATE TABLE IF NOT EXISTS notifications (
     id            INT AUTO_INCREMENT PRIMARY KEY,
     volunteer_id  INT NOT NULL,
     message       TEXT NOT NULL,
@@ -64,5 +60,5 @@ CREATE TABLE IF NOT EXISTS notifications (
     FOREIGN KEY (volunteer_id) REFERENCES volunteers(id) ON DELETE CASCADE,
     INDEX idx_volunteer_id (volunteer_id),
     INDEX idx_read (read)
-) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
-`
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci`,
+}