Migration: clear stale Metron URLs that always 404

[ajslater]

Summary

Codex's metadata view shows clickable links for identifier tags. For Metron-sourced identifiers of types genre, location, story, tag, and creditrole, those URLs (e.g. https://metron.cloud/genre/3) always 404 — Metron has no public web pages for those types, only API endpoints.

This adds a one-shot migration that clears the stale url field on existing Identifier rows matching source.name == "metron" and id_type in ("genre", "location", "story", "tag", "creditrole"). The numeric Metron key is preserved.

Pairs with comicbox PR #124, which stops generating these URLs at import time. After that PR is released and the comicbox pin is bumped, re-import will repopulate the url field where applicable (roles inherit the creator URL via comicbox's existing computed-step fallback).

Verified

• Confirmed paths 404 with curl -I: genre/3, location/3, story/3, tag/3, role/3 → 404. arc/3, character/3, creator/3, imprint/3, issue/3, publisher/3, series/3, team/3, universe/3 → 301 (still good).
• Cross-referenced with Metron's URL router — only the 9 working types have routes.
• bin/manage.py makemigrations --dry-run --check → "No changes detected"
• bin/manage.py migrate --plan lists codex.0040_clear_stale_metron_urls
• make test-python → 26 passed

Notes

• The migration uses RunPython with a no-op reverse, so it's safely reversible.
• make lint exits non-zero on this branch (and on v1.11-performance HEAD) due to a remark markdown linter complaint about an "ignored" file — pre-existing, not related to this change. Worth investigating separately.
• make test (full) requires vitest for frontend tests, which isn't installed locally — Python tests pass cleanly.

Test plan

• Run bin/manage.py migrate against a dev DB that has Metron identifiers with genre|location|story|tag|creditrole URLs and confirm the URL field clears
• Confirm the metadata view no longer renders broken Metron link icons for those tag types
• After bumping comicbox to a release containing PR v0.9.7 #124, re-import a comic and confirm role identifiers get the creator URL fallback (and the other types stay URL-less)

🤖 Generated with Claude Code