PR Description Draft: Batch Image Generation MVP

Summary

This PR adds an opt-in batch image generation MVP for Gemini image models through Sub2API.

Main capabilities:

Public async batch image API under /v1/images/batches*.
Provider support for Vertex-managed Gemini batch jobs and Gemini API batch jobs.
Redis-backed worker queue, delayed requeue, stale active recovery, and per-job locks.
PostgreSQL job/item state, provider refs kept internal, and proxied item/ZIP downloads.
Balance hold, capture, release, partial-failure settlement, and idempotent billing request ids.
Frontend user batch image guide and gated navigation entry.
Feature gates through global BATCH_IMAGE_ENABLED and group-level allow_batch_image_generation.

The feature is intentionally not GA by default. It should be enabled first through feature flag and group opt-in only.

docs/BATCH_IMAGE_MVP.md: API, lifecycle, billing, provider notes, config, official Google enablement, and operations checklist.
test-reports/batch-image-20260706-codex/test-case.md: QA case matrix.
test-reports/batch-image-20260706-codex/codex-report.md: Codex test report.
test-reports/batch-image-20260706-codex/claude-report.md: Claude Code review report plus 2026-07-07 follow-up addendum.
test-reports/batch-image-20260706-codex/smoke-summary.txt: local HTTP smoke result.

Automated/local validation recorded in the test reports:

Backend batch image service/repository/server tests: pass.
Frontend typecheck/build/full tests: pass.
Local HTTP smoke: unauthenticated access, model listing, insufficient balance, completed status/items/download, and balance restoration.
Settlement tests cover successful-image-only charging, zero-success completion, already-settled idempotency, billing crash idempotency, cost-over-hold rejection, pricing snapshot, bounded settlement retry, retry exhaustion release, and billing request ids.

Online validation recorded on 2026-07-07:

One-image Vertex success: hold 0.0804, actual 0.0737, release 0.0067, final frozen_balance=0.
Immediate cancel after submit: hold released, charged 0, no capture usage log.
Two-item partial failure: one success, one failure, charged one image only, errors.json contains failed item, final frozen_balance=0.
Gemini API-key provider path: provider selectable/callable; test key had no prepayment, so successful generation was not continued; failed submit released hold and charged 0.

No high-concurrency online stress test was run because it would create unnecessary provider cost and production pressure.
Gemini API-key upstream success still needs one paid/prepaid low-cost image test when such a key is available.
A future integration test can exercise simultaneous cancel vs settlement under load, although Redis per-job locks, PostgreSQL row locks, and billing idempotency are already present.

Merge/review behind flags only:

Keep BATCH_IMAGE_ENABLED=false by default.
Enable only for selected Gemini groups through allow_batch_image_generation=true.
Start with one controlled group and monitor job state, provider errors, hold/capture/release events, and download volume before broader enablement.