feat(identities): surface real OAuth token health in Connected accounts

[Adron]

Summary

Closes #92. Replaces #99, which GitHub auto-closed when its base branch
(fix/unlink-identity-query-param, PR #93) was deleted on merge — a closed PR whose base branch no
longer exists cannot be reopened or retargeted. Same branch, same commits, now based on main.

LinkedIdentitiesView showed each provider's /status, which reports whether the provider is
configured server-side — not whether this account's token still works. A user whose Bluesky or
Mastodon token had expired saw a healthy row and found out only when a cross-post silently failed.
POST /api/user/identities/verify is the only route that actually checks.

What's included

• verifyIdentity(provider:) — the unused providerId argument is gone; the route reads only
  provider.
• Models/IdentityHealth.swift — the outcome type, distinguishing verified, needs reconnect, and
  couldn't run the check.
• A per-identity Check connection action in LinkedIdentitiesView, with the reconnect affordance
  on rejection. Manual and per-identity by design: each call hits the third-party provider, so an
  on-appear sweep would be the wrong default.
• postCamel(_:body:mappingStatuses:) in APIClientTransport.swift — the write-side twin of the
  existing get(_:mappingStatuses:). Needed because checkResponse maps both 400 and 500 to
  APIError.server, which would have made "your token expired" and "the server fell over"
  indistinguishable — and telling those apart is one of W2: Surface OAuth token health in Connected accounts (verifyIdentity) #92's acceptance criteria.

Corrections to the issue, found by reading the route

• Success is 200 {"success":true}, not {"ok":true}; the old struct decoded a field the server
  never sends.
• There are two distinct 400s, not one — the issue's "token is bad" plus "No token to verify"
  when providerData.access_token is missing. Both are reconnect-class, so they're folded together
  deliberately.

Known backend hole — filed as #100

For any provider the route doesn't recognise, every branch is skipped, nothing is verified, and it
still stamps lastVerifiedAt and returns 200 {"success":true}. A bare "mastodon" string (no
:instance suffix) hits that path. The two 200s are byte-identical, so the client cannot compensate
and this PR does not try.

Merge note

The doc comment #90 added to the old verifyIdentity was superseded, not merged alongside — that
conflict was resolved by taking this branch's version, since keeping both would have left a stale
duplicate and a signature the call site no longer compiles against.

Testing

• xcodebuild … -parallel-testing-enabled NO -skip-testing:InterlinedListTests/E2EReadOnlyTests test
  — 1248 tests, 0 failures after merging current main.
• Tests were mutation-checked: five simultaneous mutations produced 8 failures across
  APIClientIdentitiesTests and IdentityHealthTests; restored, green.

Not verified

The live path against a genuinely revoked token — needs a real expired credential. Confirm by
revoking access at a provider, then tapping Check connection: it should flip to "Needs reconnect",
while airplane mode on a healthy row should instead read "Couldn't run the check just now."

🤖 Generated with Claude Code