Make Slack IDs unique but Slack usernames nonunique

[MMK21Hub]

• Slack usernames are no longer unique
  • Only affects legacy DBs
  • Fixes Some users' threads get ignored (UniqueViolationError) #199
• Ensure Slack IDs are unique
  • Only affects new DBs since legacy ones already have a unique index
• Database migrations, yippee

[Copilot]

Pull request overview

This PR addresses Slack user identity constraints to prevent UniqueViolationError during user upserts by making Slack IDs unique while allowing Slack usernames to be non-unique (to support legacy Prisma DB edge-cases like username “swaps”).

Changes:

• Remove legacy uniqueness on User.username (via migration) and write the internal Slack name into User.username.
• Enforce uniqueness on User.slackId (schema + migration).
• Add a small raw-migration helper to reduce boilerplate for SQL-only migrations.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
nephthys/utils/slack_user.py	Adds username() accessor and changes display-name fallback behavior.
nephthys/events/message_creation.py	Stores Slack internal username (not display name) into User.username on upsert.
nephthys/database/tables.py	Marks User.slack_id as unique at the Piccolo schema level.
nephthys/database/raw_migration.py	Adds a helper to build raw SQL migrations consistently.
nephthys/piccolo_migrations/nephthys_2026_05_07t10_40_49_usernames_not_unique.py	Drops the legacy unique index on User.username.
nephthys/piccolo_migrations/nephthys_2026_05_07t11_04_02_unique_slack_id.py	Adds a unique index on User.slackId.

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