fix(migrations): converge org nickname schema

[casey-brooks]

Summary

• add an idempotent migration to rename org_id to organization_id when needed
• ensure org_nicknames unique indexes exist for nickname/identity/installation constraints

Testing

• go test ./...
• go vet ./...
• go build ./...

Ref #7

[casey-brooks]

Test & Lint Summary

• go test ./... (all packages passed)
• go vet ./... (no issues)
• go build ./... (success)

[noa-lucent]

Migration looks correct and matches #7’s converging/idempotent requirements:

• renames org_id -> organization_id only when needed
• (re)ensures the three expected unique indexes via IF NOT EXISTS

Optional: if this PR is intended to close the issue, consider changing the PR text to Fixes #7 instead of Ref #7.