fix: update MySQL logic to use 0 for non-pinned posts instead of NULL

[Alam-2U]

Description

After migrating data from MongoDB to MySQL, the pinned column in the forum_commentthread table had inconsistent values:

• Migrated posts: 0 (non-pinned) and 1 (pinned)
• Newly created posts: NULL for non-pinned

Since sorting prioritizes the pinned column before last_activity_at, this inconsistency caused incorrect ordering. Non-pinned migrated posts (0) were always appearing above newly created non-pinned posts (NULL), regardless of their recent activity (1 → 0 → NULL).

Root Cause

Newly created posts were not assigning a default value to the pinned column, resulting in NULL values instead of 0 for non-pinned posts.

Changes Made

• Updated logic to set pinned = 0 for non-pinned posts instead of NULL

Post Task

• Backfill existing records to replace NULL values with 0
• Enforce schema constraint: pinned TINYINT(1) NOT NULL DEFAULT 0

Ticket

COSMO2-902

[Copilot]

Pull request overview

Fixes inconsistent pinned values in the MySQL CommentThread model so newly created non-pinned threads use 0/False instead of NULL, preventing incorrect ordering when sorting by pinned-first.

Changes:

• Make CommentThread.pinned default to False (and update MySQL creation logic accordingly).
• Update MongoDB→MySQL migration helper to default missing pinned values to False.
• Adjust MySQL model test expectations and bump package version.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/test_backends/test_mysql/test_models.py	Updates thread creation test to expect pinned=False rather than None.
forum/migration_helpers.py	Defaults missing pinned values to False during thread migration/update.
forum/backends/mysql/models.py	Changes pinned field to default to False and removes nullable typing/field config.
forum/backends/mysql/api.py	Forces pinned=False on MySQL thread creation to avoid NULL inserts.
forum/__init__.py	Bumps version to 0.5.8.

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