fix(backend): 修复全面审阅发现的核心问题

修复索引首次失败回滚、Vault 扫描边界、Markdown 代码围栏和 UTF-16 Citation 偏移。

收紧 Plugin 权限与 JSON Schema 校验，补齐 Note Move、Task、Attachment 和 Transcript Tool。

接入 OpenAI SSE 与 Ollama JSONL 真流式输出，修正 Provider PATCH 语义并限制运行时内存保留。

新增对应回归测试，后端测试增至 62 项。

backend/app/services/transcription_service.py

@@ -0,0 +1,40 @@
+from __future__ import annotations
+
+from collections import OrderedDict
+from datetime import datetime, timezone
+from pathlib import Path
+from uuid import uuid4
+
+from app.contracts import TranscriptionJob
+from app.services.attachment_service import attachment_path
+
+_jobs: OrderedDict[str, TranscriptionJob] = OrderedDict()
+MAX_JOBS = 100
+
+
+def create_transcription(attachment_id: str, language: str | None = None) -> TranscriptionJob:
+    del language  # 预生成 transcript 暂不需要语言识别。
+    source = attachment_path(attachment_id)
+    transcript = source if source.suffix.lower() in {".txt", ".md"} else Path(f"{source}.txt")
+    job = TranscriptionJob(
+        job_id=f"transcription_{uuid4().hex}",
+        attachment_id=attachment_id,
+        status="completed" if transcript.is_file() else "failed",
+        text=transcript.read_text(encoding="utf-8") if transcript.is_file() else None,
+        error_code=None if transcript.is_file() else "TRANSCRIPTION_BACKEND_UNAVAILABLE",
+        error_message=(
+            None
+            if transcript.is_file()
+            else "No host-generated transcript is available; local speech models are phase two."
+        ),
+        created_at=datetime.now(timezone.utc),
+    )
+    _jobs[job.job_id] = job
+    while len(_jobs) > MAX_JOBS:
+        _jobs.popitem(last=False)
+    return job.model_copy(deep=True)
+
+
+def get_transcription(job_id: str) -> TranscriptionJob | None:
+    job = _jobs.get(job_id)
+    return job.model_copy(deep=True) if job else None