Follow-ups to #257: suggestion-hides-unapproved edge, hiddenChannels privacy test, cache-copy and TTL test gaps #276

Follow-ups from pve-agent3's round-2 review of #257 (#251, merged as c6b356de). Details and evidence are in the review comment on #257.

1. Semantics (minor): an anonymous suggestion can hide a channel the admin never acted on. A channel with stored history whose name the ingestor no longer decrypts through config is listed on master before fix(channels): hide revoked shared channels from the channel list (#251) #257. Examples are a name removed from hashChannels/channelKeys by a hot reload, or a name past the 4096-name cap of builtin-channels.json. After fix(channels): hide revoked shared channels from the channel list (#251) #257, an anonymous suggestion for that name (pending) hides it. Decide on one of these and lock it with a test:
   • hide only names whose proposal has ever been approved (needs history);
   • or document the trade-off.
2. Test gap: the cache-safety copy in hideRevoked on the default path. TestRevokedFilterDoesNotMutateCacheAndKeepsEncrypted only covers ?includeEncrypted=true, where the slice is already copied. Add the default request, where hideRevoked receives GetChannels' cached slice. The mutant that drops the copy must fail.
3. Test gap (privacy-relevant): the EXISTS (… transmissions …) clause. Without it, hiddenChannels on the public /api/channels would list proposals that have no traffic, including pending suggestions, which are otherwise admin-only. Add a test that a pending suggestion without traffic never appears in hiddenChannels. Mutant G1 must fail.
4. Test gap: invalidateApiCache('/channels') in refreshChannelList. The harness api() has no TTL cache, so mutant F3 survives. The same gap exists for onApproved. Add a harness with a TTL cache, or an E2E that revokes and checks the list without waiting 15 s.
5. Docs: hiddenChannels is global, not per region, and another open tab keeps its old hidden set until it reloads. That also applies after a re-approval. Add one sentence about it.