fix: retry cloud model catalog sync when first account becomes active

[clarezoe]

Problem

fetchAndMergeModelCatalog() was only called once at startup. If no active account existed at that moment, the cloud catalog sync was skipped entirely and never retried — meaning newly available upstream models (e.g. Claude Fable 5, redeployed 2026-07-01) wouldn't appear in /v1/models until a manual restart with an already-active account.

Root Cause

In src/auth.js, fetchAndMergeModelCatalog() runs once during initAuth(). It finds the first active account and, if none exists, logs 'No active account for model catalog fetch' and returns. There is no retry path — accounts added later never trigger a re-sync.

Fix

Add a _modelCatalogSynced flag and a trySyncModelCatalog() fire-and-forget wrapper that coalesces concurrent calls. Call it from every account activation path:

• addAccountByKey — adding API key account
• addAccountByToken — adding token account
• addAccountByEmail — email login
• setAccountStatus(id, 'active') — manual status change
• resetAccountErrors — error reset recovery
• reportSuccess — auto-recovery on successful request
• maybeRecoverErrorAccount — half-open recovery

trySyncModelCatalog() checks:

1. If sync already succeeded (_modelCatalogSynced) → skip
2. If a sync is already in flight (_modelCatalogSyncPromise) → skip
3. If no active account exists → skip
4. Otherwise → fire fetchAndMergeModelCatalog() in the background

Verification

Tested on production deployment. After restart with no active accounts at startup, adding the first account triggered the sync automatically:

Model catalog: 150 cloud models, 45 new entries merged

Claude Fable 5 variants (claude-5-fable-low/medium/high/xhigh/max) now appear in /v1/models.