fix(migrations): use INTEGER for sessions.user_id to match users.id FK type

[dkhalife]

Problem

Migration 7 (sessions) fails on MySQL/MariaDB with:

```nError 1005 (HY000): Can't create table tasks.`sessions` (errno: 150 "Foreign key constraint is incorrectly formed")

On an affected deployment, `users.id` is `bigint(20)` (MariaDB), while migration 7 declared `sessions.user_id` as `INT` — type mismatch causes the FK to be rejected.

## Fix

- Use `INTEGER` for `sessions.id` and `sessions.user_id` (matches `users.id` declaration in migration 1, same pattern used by `labels.created_by` and `tasks.created_by` which FK successfully).
- Name the FK constraint (`fk_users_sessions`) for parity with other tables.
- Create indexes as separate statements after `CREATE TABLE` (matches the working `app_tokens` migration pattern).

No schema changes for SQLite path.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 23 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
apiserver/internal/migrations/007_sessions.go	0.00%	23 Missing ⚠️

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[Copilot]

Pull request overview

Updates migration 007 (sessions table) to improve MySQL/MariaDB compatibility by aligning column declarations and DDL structure with the patterns used elsewhere in the migration set.

Changes:

• Refactors the MySQL sessions migration to execute CREATE TABLE plus index creation as separate statements.
• Adjusts MySQL column types for sessions.id / sessions.user_id and names the users FK constraint (fk_users_sessions).

Show a summary per file

File	Description
apiserver/internal/migrations/007_sessions.go	Refactors MySQL sessions table creation, adds explicit FK constraint name, and creates indexes via separate statements.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 1