Re-protecting a synced private feed address leaves a new secret ID each time #490

When a synced data.json holds a raw private feed address (a device still on 2.21.1 wrote it), every read that protects the address stores it under a fresh podnotes-feed-url-<uuid> secret (src/services/FeedUrlRepository.ts, store). One incoming change can be read several times (the adoption read, the fresh read before the protective save from #489, and overlapping notifications). Only the last reference reaches data.json; the rest are orphans.

Found by the independent verification of #489 on Obsidian 1.14.4 (Linux):

• A one-minute simulation with 20 incoming raw changes grew the PodNotes secrets from 12 to 70.
• After a plugin reload, the startup sweep cleared all but one value, but listSecrets() still returned all 70 IDs. SecretStorage has no delete, so a cleared ID stays listed.

So during the 2.22.0 upgrade window, a device that syncs with a 2.21.1 device collects secret IDs without bound, and they survive restarts.

Expected: protecting the same address again reuses its reference, so the number of PodNotes secret IDs stays bounded by the number of distinct private feeds.

Related: #488, #489.