BB-888: feat: add built-in identifier validation with admin dropdown

[atharvsp02]

Problem

Ticket: BB-888
BookBrainz validates identifiers using only regex, which cannot catch mathematical errors like invalid ISBN checksums. A typo like 9780306406150 (wrong check digit) passes regex validation and gets saved to the database.

Solution

Added support for built-in JavaScript validation functions that run alongside regex:

• New validation_function column in identifier_type table maps to named JS functions
• ISBN-10 and ISBN-13 checksum validators implemented as first built-in validators
• Admin UI gets a dropdown to assign validation functions to identifier types

Adding support for new identifier checksums (like ISSN etc.) just requires writing a validation function in identifier-validators.ts and registering it. Once deployed, admins can assign it to any identifier type through the UI without further code changes.

Areas of Impact

• identifier-validators.ts - ISBN checksum logic and validator registry
• common.ts - Calls built-in validator before falling back to regex
• identifier-type.tsx - Admin dropdown for validation function
• typeUtils.tsx, types.ts` - Added validationFunction property
• typeRouteUtils.ts - Server side save/load of new field
• identifier-types-tree.tsx - Display validation function in type listing
• bookbrainz.sql - Added column to base schema
• sql/migrations/2026-03-23/ - Migration for new column

• I have run the code and manually tested the changes

Screenshots

Before

• Invalid checksum passes with regex only

After

• Added validation function dropdown to Identifier type editor

• Invalid checksum now correctly rejected

AI usage

• I did not use any AI
• I have used AI in this PR (add more details below)

If you did use AI:

• I used AI tools for communication
• I used AI tools for coding
• I understand all the changes made in this PR

Action

Run SQL migration

1. Run SQL migration sql/migrations/2026-03-23/up.sql
2. Admins can assign validators to ISBN types via the admin UI after deployment