fix: prevent duplicate label creation from Android sync engine

[dkhalife]

Problem

Creating a label from the Android app resulted in many duplicate copies being created.

Root Cause

Response format mismatch (primary): The server's CreateLabel endpoint returned {"label": {"id": ..., "name": ..., "color": ...}} (a full object), but the Android client's LabelCreatedResponse expected {"label": 123} (just an integer ID, matching the task creation pattern). Gson failed to deserialize the nested object as an Int, so response.body()?.label was null, the outbox recorded a failure ("Empty create response"), and retried on every sync cycle -- each retry creating another copy on the server.

No uniqueness enforcement (secondary): There was no unique constraint on (created_by, name) in the labels table and no server-side duplicate check, so every retry succeeded in creating another label.

Changes

Server (apiserver)

• Fix response format: CreateLabel now returns {"label": id} (just the ID), consistent with task creation. The WebSocket broadcast still sends the full label object for real-time sync.
• Migration 008: Deduplicates existing labels (keeping lowest ID, reassigning task_labels), then adds unique index on labels(created_by, name). Supports both SQLite and MySQL.
• Duplicate checking: CreateLabel and UpdateLabel now check for existing labels with the same name (per user) before insert/update, returning HTTP 409 Conflict. DB constraint violations are also caught and mapped to 409.
• GORM model: Updated Label struct tags with uniqueIndex for documentation.

Frontend

• Updated CreateLabel API to use LabelIdResponse ({label: number}) and reconstruct the full label from request data + returned ID in the Redux slice (matching the existing task creation pattern).

[codecov]

Codecov Report

❌ Patch coverage is 5.60748% with 101 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...rver/internal/migrations/008_unique_label_names.go	0.00%	68 Missing ⚠️
apiserver/internal/services/labels/label.go	0.00%	31 Missing ⚠️
apiserver/internal/repos/label/label.go	75.00%	1 Missing and 1 partial ⚠️

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[Copilot]

Pull request overview

This PR fixes duplicate label creation caused by a client/server response-shape mismatch during label creation, and adds server-side + DB-level enforcement to prevent same-user duplicate label names.

Changes:

• Align label creation API response to return { label: id } (matching task creation), and update the frontend to reconstruct the label object from the request + returned id.
• Add server-side duplicate-name checks for label create/update and map duplicates to HTTP 409 Conflict (including DB-constraint violations).
• Add migration 008 to deduplicate existing labels and introduce a unique index on (created_by, name) for both SQLite and MySQL.

Show a summary per file

File	Description
frontend/src/store/labelsSlice.ts	Reconstructs label object from returned label id during create, matching existing task-create pattern.
frontend/src/api/labels.ts	Updates CreateLabel HTTP transport typing to expect { label: number }.
apiserver/internal/services/labels/label.go	Adds duplicate checks + 409 mapping; changes CreateLabel response format to { label: id }.
apiserver/internal/repos/label/label.go	Adds LabelExistsByName helper used by the service layer.
apiserver/internal/models/label.go	Documents uniqueness intention via GORM tags for (created_by, name).
apiserver/internal/migrations/008_unique_label_names.go	Deduplicates existing labels and adds unique index on (created_by, name) across SQLite/MySQL.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 3