feat: implement full-platform STT API (v2.3 consensus plan)

Batch+realtime transcription API: faster-whisper engine w/ EngineOwner
(single GPU owner, OOM downgrade chain, persisted attempted_profiles),
hardware-adaptive device manager (T0-T3 VRAM tiers), Redis/in-proc job
queue w/ leases + crash recovery, postprocess (glossary/rules/LLM w/
egress guard), privacy-first result store (UUID keys, source deleted
after transcribe), retention sweeper, API-key auth (HMAC digests,
scopes, job ownership), WebSocket realtime lane (LocalAgreement),
CLI (detect/transcribe/bench/serve/key), Docker, benchmark runner.

127 mock-based tests pass; ruff clean. Includes verification checklist
and autoplan review notes.

tests/unit/test_formats.py

@@ -0,0 +1,85 @@
+"""포맷 렌더링 (json/txt/srt/vtt) + 결과 스키마 계약 테스트."""
+
+from __future__ import annotations
+
+import json
+
+import pytest
+
+from luke_scribe.results.formats import render
+from luke_scribe.results.models import TranscriptResult
+
+
+@pytest.fixture
+def result(transcript_result: dict) -> TranscriptResult:
+    return TranscriptResult.model_validate(transcript_result)
+
+
+class TestFormats:
+    def test_json_roundtrip(self, result: TranscriptResult):
+        out = render(result, "json")
+        data = json.loads(out)
+        assert data["schema_version"] == "1.0"
+        assert data["segments"][0]["text"].startswith("오늘")
+        assert data["text"]  # 계약: text = segment 텍스트 연결
+
+    def test_txt(self, result: TranscriptResult):
+        out = render(result, "txt")
+        assert "오늘 API 서버에서" in out
+        assert out.endswith("\n")
+
+    def test_srt(self, result: TranscriptResult):
+        out = render(result, "srt")
+        assert "1" in out
+        assert "00:00:00,000 --> 00:00:02,500" in out
+        # 세그먼트 순번 + 타임스탬프 + 텍스트
+        assert "오늘 API 서버에서" in out
+
+    def test_vtt(self, result: TranscriptResult):
+        out = render(result, "vtt")
+        assert out.startswith("WEBVTT")
+        assert "00:00:00.000 --> 00:00:02.500" in out
+
+    def test_unsupported_format(self, result: TranscriptResult):
+        with pytest.raises(ValueError):
+            render(result, "xml")  # type: ignore[arg-type]
+
+    def test_empty_segments_srt(self):
+        r = TranscriptResult(status="completed", segments=[], text="")
+        out = render(r, "srt")
+        assert out == ""
+
+
+class TestSchemaContract:
+    def test_status_completed_no_error(self, transcript_result: dict):
+        result = TranscriptResult.model_validate(transcript_result)
+        assert result.error is None
+
+    def test_failed_gets_default_error(self):
+        r = TranscriptResult(status="failed", text="")
+        assert r.error is not None
+        assert r.error.code == "transcription_failed"
+
+    def test_cancelled_gets_default_error(self):
+        r = TranscriptResult(status="cancelled", text="")
+        assert r.error is not None
+        assert r.error.code == "cancelled"
+
+    def test_completed_error_rejected(self):
+        with pytest.raises(ValueError):
+            TranscriptResult(status="completed", error={"code": "x", "message": "y"}, text="")
+
+    def test_segment_index_must_be_nonneg(self):
+        with pytest.raises(ValueError):
+            TranscriptResult(
+                status="completed", segments=[{"index": -1, "start": 0, "end": 1, "text": "x"}]
+            )
+
+    def test_duration_gt_zero(self):
+        with pytest.raises(ValueError):
+            TranscriptResult(
+                status="completed",
+                normalized_audio={"duration_sec": 0},
+                segments=[],
+                text="",
+            )