Route limited mailboxes through opt-in UIDONLY acquisition

[gabeosx]

Stack position

• Protocol prerequisite: Support opt-in UIDONLY acquisition for large Yahoo mailboxes rustmailer/bichon#337
• Storage prerequisite: Add canonical UID-scoped storage primitives #1

This final draft exists in the fork so GitHub shows only acquisition, gate, and UI changes. It should be retargeted/reopened upstream after the storage prerequisite merges.

Scope

• default-off per-account UIDONLY setting in the API and Web account form
• exact capability routing for UIDONLY + PARTIAL + MESSAGELIMIT
• bounded, checkpointed acquisition for Yahoo's capped mailbox view
• ordinary IMAP remains on the existing path, including its empty-mailbox fast path and previous literal-size configuration behavior

Automatic reconciliation with records from an earlier capped Yahoo import is intentionally excluded. Disabling the setting takes effect on the next synchronization; mid-run revocation is not part of this change.

Verification

• cargo check --workspace
• Web production build
• 38 focused UIDONLY/acquisition/storage tests
• all 61 blob unit, durability, fuzz, and integration tests
• disposable Cyrus 3.12: 2 exact messages, restart with zero body refetch, cleanup verified
• bounded Yahoo: one metadata-only connection plus one sample connection, at most 10 inventory records, one body capped at 25 MiB, no mutations