Implement optimized label feature with batched queries

[Copilot]

Adds label functionality with N+1 query optimization built in. Previously would require N+1 queries (one per task) to fetch labels; now uses single JOIN query to fetch labels for all tasks.

Database Schema

• labels table: id, name (unique), color, created_at
• task_labels junction table: composite PK on (task_id, label_id), foreign keys with CASCADE
• Indexes on both task_id and label_id for JOIN performance

Core Optimization

getTaskLabelsOptimized() batches label fetching:

// Single query fetches labels for all tasks at once
const getTaskLabelsOptimized = (taskIds, callback) => {
  const sql = `
    SELECT tl.task_id, l.id, l.name, l.color, l.created_at
    FROM task_labels tl
    INNER JOIN labels l ON tl.label_id = l.id
    WHERE tl.task_id IN (${placeholders})
  `;
  // Group by task_id in memory for O(1) lookup
};

enrichTasksWithLabels() helper applies this pattern to /api/tasks and / routes, reducing queries from N+1 to 2 total (1 for tasks, 1 for all labels).

API Endpoints

• POST /api/labels - create label (validates hex color, enforces unique names)
• GET /api/labels - list all labels
• POST /api/tasks/:id/labels - assign label(s), supports {labelId: 1} or {labelIds: [1,2,3]}
• DELETE /api/tasks/:id/labels/:labelId - remove label
• POST /api/tasks/bulk/labels - bulk assign via {assignments: [{taskId, labelId}, ...]}
• GET /api/tasks - now includes labels array per task

Implementation Notes

• Bulk operations use callbackCalled flag to prevent race condition in async forEach loops
• Express route ordering: /api/tasks/bulk/labels placed before /api/tasks/:id/* to avoid param matching
• No explicit transactions (sqlite3 node API callback model makes nested transactions error-prone)
• Validation accepts both numeric and string IDs for flexibility

Original prompt

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This section details on the original issue you should resolve

<issue_title>new label feature optimization</issue_title>
<issue_description>## Overview\n\nThis issue tracks optimization opportunities for the label feature in the Task Manager app.\n\n## Areas to Optimize\n\n- enrichTasksWithLabels helper: Currently makes N individual getTaskLabels DB calls (one per task). This should be batched into a single query to reduce database round-trips.\n- Label assignment: Bulk assign/remove labels to tasks in a single transaction instead of individual calls.\n- Caching: Consider caching frequently accessed labels to avoid redundant DB reads.\n\n## Expected Outcome\n\n- Reduced database queries when loading tasks with labels\n- Improved response times for /api/tasks and / endpoints\n- Cleaner, more scalable label-fetching logic</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes new label feature optimization #12

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