Implement Issue #2: Scheduling & Publishing

Replaces stub schedule CRUD with full shift template + instance system.

- DB: add shift_templates, shift_template_roles, shift_template_volunteers,
  shift_instances, shift_instance_volunteers tables
- schedule package: ShiftTemplate and ShiftInstance models with store
  (generate, publish/unpublish, per-instance edits, volunteer confirmation)
- API: shift-templates CRUD + shifts generate/publish/unpublish/update/confirm
- Notifications sent on publish (FR-S04), unpublish (FR-S05), instance edit
  (FR-S09), and volunteer added mid-month (FR-S10)
- Frontend: Schedules page with month navigation, template management,
  publish/unpublish controls, and per-shift edit/confirm
- Tests: Go handler tests (14 cases) + React tests (11 cases)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/db/schema.go

@@ -77,5 +77,62 @@ var statements = []string{
     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`,
+	`CREATE TABLE IF NOT EXISTS shift_templates (
+    id           INT AUTO_INCREMENT PRIMARY KEY,
+    name         VARCHAR(255) NOT NULL,
+    day_of_week  TINYINT NOT NULL COMMENT '0=Sunday 1=Monday ... 6=Saturday (matches Go time.Weekday)',
+    start_time   TIME NOT NULL,
+    end_time     TIME NOT NULL,
+    min_capacity INT NOT NULL DEFAULT 1,
+    max_capacity INT 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 shift_template_roles (
+    id          INT AUTO_INCREMENT PRIMARY KEY,
+    template_id INT NOT NULL,
+    role_name   VARCHAR(255) NOT NULL,
+    count       INT NOT NULL DEFAULT 1,
+    FOREIGN KEY (template_id) REFERENCES shift_templates(id) ON DELETE CASCADE,
+    INDEX idx_template_id (template_id)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci`,
+	`CREATE TABLE IF NOT EXISTS shift_template_volunteers (
+    id           INT AUTO_INCREMENT PRIMARY KEY,
+    template_id  INT NOT NULL,
+    volunteer_id INT NOT NULL,
+    UNIQUE KEY uq_template_volunteer (template_id, volunteer_id),
+    FOREIGN KEY (template_id) REFERENCES shift_templates(id) ON DELETE CASCADE,
+    FOREIGN KEY (volunteer_id) REFERENCES volunteers(id) ON DELETE CASCADE
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci`,
+	`CREATE TABLE IF NOT EXISTS shift_instances (
+    id           INT AUTO_INCREMENT PRIMARY KEY,
+    template_id  INT NULL,
+    name         VARCHAR(255) NOT NULL,
+    date         DATE NOT NULL,
+    start_time   TIME NOT NULL,
+    end_time     TIME NOT NULL,
+    min_capacity INT NOT NULL DEFAULT 1,
+    max_capacity INT NOT NULL DEFAULT 1,
+    status       VARCHAR(20) NOT NULL DEFAULT 'draft' COMMENT 'draft or published',
+    year         INT NOT NULL,
+    month        INT NOT NULL,
+    created_at   TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    updated_at   TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+    FOREIGN KEY (template_id) REFERENCES shift_templates(id) ON DELETE SET NULL,
+    INDEX idx_year_month (year, month),
+    INDEX idx_status (status)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci`,
+	`CREATE TABLE IF NOT EXISTS shift_instance_volunteers (
+    id           INT AUTO_INCREMENT PRIMARY KEY,
+    instance_id  INT NOT NULL,
+    volunteer_id INT NOT NULL,
+    confirmed    TINYINT NOT NULL DEFAULT 0,
+    confirmed_at DATETIME NULL,
+    UNIQUE KEY uq_instance_volunteer (instance_id, volunteer_id),
+    FOREIGN KEY (instance_id) REFERENCES shift_instances(id) ON DELETE CASCADE,
+    FOREIGN KEY (volunteer_id) REFERENCES volunteers(id) ON DELETE CASCADE,
+    INDEX idx_instance_id (instance_id),
+    INDEX idx_volunteer_id (volunteer_id)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci`,
 }