Add Indices to Foreign Key Columns in SubscribedSubredditData, SubscribedUserData and MultiReddit

[adripo]

Description:

Problem:

In the current implementation, the username column in the SubscribedSubredditData, SubscribedUserData, and MultiReddit tables references a foreign key but is not indexed. This may trigger full table scans whenever the parent table is modified, leading to performance issues.
Details: https://github.com/Docile-Alligator/Infinity-For-Reddit/actions/runs/10067294236/job/27830465944#step:4:109

Solution:

To resolve this, indices have been added to the username columns in the database tables. Additionally, the database schema version has been incremented and migration strategies have been provided to handle the creation of these indices.

Testing:

• Verified that the indices are created successfully during migration.
• Ensured that the application runs without performance warnings.

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Checklist:

• Added @Index annotation to username columns.
• Incremented the database version.
• Provided migration strategies.
• Verified successful migration and absence of performance warnings.

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Please review and merge the changes.