fix: canonicalize subscription urls and deduplicate records

[Priveetee]

Summary

• canonicalize subscription channel URLs on write and delete paths using a shared URL canonicalizer
• add startup migration to canonicalize and deduplicate existing subscriptions rows per (user_id, channel_url) while keeping latest subscribed_at
• cover route behavior and migration behavior with dedicated tests

Validation

• ran full test suite with ./gradlew test
• result: success