修复空转录成功缓存及撤权后的摘要请求

[zj1123581321]

Fixes #243

This PR prevents empty transcripts from being treated as successful cache entries or handed to downstream consumers.

• FunASR results with no usable formatted text raise the existing EmptyTranscriptError before cache or handoff.
• CapsWriter output that is successfully read as empty or whitespace-only is rejected.
• YouTube API subtitles fail when parsed text and text rebuilt from preserved segments are both empty.
• After a successful share-revocation DELETE, a pending hover timer for that still-rendered row checks that its view token remains present before requesting a summary. Timers for other rows remain active.

Valid numeric FunASR text, numeric subtitles preserved in structured segments, existing valid cache rows, and the revoked-token 404 contract remain covered.

Accepted P2: if opening or reading an already-written CapsWriter .txt file raises an I/O error, the existing reader logs a warning and may continue with an empty value. This PR covers empty or whitespace-only producer output and successfully-read bodies, not that separate read-I/O failure path. Already-issued summary requests are not aborted. A detached row that retains data-vt after an unrelated list redraw can still send its existing pending request; this existing behavior is unchanged.

Validation:

• uv run pytest -q tests/integration/test_issue243_empty_transcript.py — 10 passed.
• uv run pytest -q tests/unit/test_capswriter_contract.py — 24 passed.
• Cross-row Chromium regression: the old global timer cleanup expected 1 summary GET for active row B but received 0; the repaired test passes with B's unique 200 response and tooltip while revoked row A receives no request.
• Full-chain Chromium regression after owner revoke — 1 passed. Removing only the row-token guard makes the post-DELETE assertion red (Expected 0 summary GETs / Received 1); the guard was restored byte-for-byte.
• uv run pytest -q tests/integration/test_local_upload_lifecycle.py::test_public_read_summary_exports_and_owner_history_close_without_content_bypass — 1 passed.

[github-actions]

Required Gate v2 — 状态面板

当前状态：pass · 可合并

当前裁决：code_pass / primary_pass

PR 累计主审轮（影子）：3/5 · collecting

Gate 历史（v1；来源为持久化 gate_terminal 制品）

Run	Attempt	Head	状态	PR 累计主审轮（影子）	收件人动作
39003390059	1	250e8e9	skipped	—	无需动作（主审未跑，绿≠过审）
39005559770	1	5eab9f1	fail	—	要修代码
39005579872	1	5eab9f1	pass	—	可合并
39006414418	1	c29326f	fail	—	要修代码
39007647198	1	2b99891	pass	3/5 · collecting	可合并

历史行按 run_id + run_attempt 去重并只增不删；删除本评论后可由 gate_terminal 制品重建。