feat(usage): streaming recursive directory usage API

[pengpeng]

Summary

Adds a streaming GET /api/usage/*path endpoint that reports recursive file count and total byte size for a directory (and (1, size) for a single file), reusing the archive entries NDJSON streaming code and wire protocol.

• Shared NDJSON writer: extracted pkg/hertz/biz/handler/stream (Emit / Done / Fail, owning content-type, per-line flush, and the terminal {"_done":...} / {"_error":...,"code":...} framing). Rewired archive EntriesMethod to use it so both endpoints share one code path.
• Driver interface: added DirUsage(ctx, contextArgs, emit) to base.Execute, mirroring archive's synchronous walk-with-callback style:
  • posix: filepath.WalkDir summing regular files with throttled progress; cache/external delegate.
  • cloud: new rclone GetFilesUsage via operations/size (single server-side recursive call returning count+bytes).
  • sync: repo root uses Seafile size/file_count fast path, files resolve size from the parent listing, subdirs walk level-by-level via getFiles.
• Endpoint: new usage handler + router (reusing ResolveFileHandler, Gate on ActionRead, and the shared stream writer), registered in register.go.

Wire protocol: zero or more {"count":N,"size":B} progress lines, then a terminal {"_done":true,"count":N,"size":B} (or {"_error":"...","code":"..."}). The client closing the connection cancels the walk via ctx.

The matching frontend (file attributes dialog showing live size + item count) lives in the Olares repo.

Test plan

• posix dir: streams incrementing count/size, terminal totals match du-style sum of regular files
• posix single file: returns count=1 and correct size
• cloud (google/s3/tencent/dropbox) dir: returns count+bytes from operations/size
• sync repo root: fast path returns repo file_count/size; subdir recurses correctly
• client disconnect mid-stream cancels the walk (code: "canceled")
• permission denied returns 403 before streaming starts
• archive /entries endpoint still streams unchanged (shared writer regression)

Made with Cursor

[pengpeng]

Heads-up on the red test check: it is a pre-existing failure inherited from main, not caused by this PR.

• Failing test: TestHydrateExternalRootItemMetadataMountedValid (pkg/drivers/posix/posix/posix_list_test.go:117), mod time mismatch: got=0001-01-01 ... expected=2026-06-01.
• It goes through hydrateExternalRootItemMetadata -> statMountedExternalRootItem -> externalMountGuard.run(...), which is unrelated to the DirUsage / usage-streaming changes here.
• The same test already fails on main: the CI run for the PR Decouple external mount probing from guarded operations #344 merge (Decouple external mount probing from guarded operations, commit ea294d4) failed with the identical test (run 27017151726). That change made the guard return "probe pending" for an un-probed mount, so statMountedExternalRootItem no longer runs the stat and ModTime stays zero, but posix_list_test.go was not updated to the new model.
• This branch is already rebased on the latest main (including ea294d4); the failure persists because the test itself is stale.

This PR's own packages (drivers / stream / usage handler+router) build and go vet clean under GOOS=linux. Suggest fixing TestHydrateExternalRootItemMetadataMountedValid on main separately (owned by the #344 change); this PR will go green once that lands and we rebase.