Remove unused access_tokens table

[Copilot]

The access_tokens table was part of the initial schema but is unused. Authentication is now JWT-based with tokens validated but not stored in the database.

Changes

• Created migration 1767972708_remove_access_tokens to drop the table
• Down migration recreates full table structure (columns, foreign keys, indexes, trigger) for rollback capability

Original prompt

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This section details on the original issue you should resolve

<issue_title>Remove access token registry</issue_title>
<issue_description>Users sign in using a JWT token, tokens should not be stored within the access token registry. We should remove/cleanup the database tables.</issue_description>

<agent_instructions>The access_token table is not being used anymore in nexus. Please create a migration to remove this table.</agent_instructions>

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

• Fixes Remove access token registry #35

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