Make GitHub numeric ID the sole auth identity, drop username-based auth lookup

[madebygps]

Background

users.github_username was originally proposed as a NOT NULL change in #448 (Tier 2 item 1). On audit, that change is unsafe because of an underlying design issue: we authenticate users by their mutable display name (github_username) instead of their immutable numeric ID (users.id, which is already the GitHub ID).

The smell

get_or_create_user_from_github() does:

1. Look up the existing owner of the username
2. If a different user already has it (because they renamed away on GitHub), call clear_github_username() to set the old owner's github_username to NULL
3. Assign the username to the new owner

This means a real user account can silently lose its display name when another user renames into their old handle. Next time the original user logs in, they can no longer be looked up by username (their row is now github_username = NULL).

Perfect-world fix

Use GitHub's immutable numeric ID for auth lookups. Treat github_username as display-only, refreshed on every login.

Schema changes

• Drop UniqueConstraint("github_username", name="uq_users_github_username")
• Add NOT NULL on github_username
• Optionally add a non-unique index on github_username for admin search

Code changes

• get_or_create_user_from_github() looks up by user_repo.get_by_id(github_id) instead of get_by_github_username(...)
• Delete clear_github_username() repository method and service-level conflict handling
• Update tests in api/tests/services/test_users_service.py (drop the username-conflict test path)

Migration

1. Preflight: SELECT COUNT(*) FROM users WHERE github_username IS NULL. Backfill or delete those rows.
2. Drop the unique constraint.
3. Add NOT NULL.

Acceptance criteria

• Username collision on GitHub rename no longer mutates other user rows.
• github_username is NOT NULL in the schema.
• Auth lookup uses github_id only.
• clear_github_username() and its callers are deleted.

Source

Audit finding from #448 (item 1). Replaces that item with a proper-scope fix.