refactor(api): TOML 配置 SSOT、统一错误契约、Auth/事务加固与可观测性 (#33)

配置 SSOT（TOML + .env）
统一错误契约
Auth 与事务边界
Redis / Celery 可靠性:业务 Redis（DB/0）与 Celery broker/backend（DB/1）显式拆分；连接池、sync client
可观测性（OpenTelemetry + LGTM）

api/app/features/story/service.py

@@ -10,6 +10,8 @@ from datetime import datetime, timezone
 
 from sqlalchemy.ext.asyncio import AsyncSession
 
+from app.core.db import transactional
+from app.core.errors import NotFoundError
 from app.core.logging import get_logger
 from app.features.memoir import repo as memoir_repo
 from app.features.memoir.asset_resolver import strip_asset_image_refs_from_markdown
@@ -124,45 +126,46 @@ class StoryService:
     ) -> str:
         """Create story, commit, return story_id."""
         md = strip_asset_image_refs_from_markdown(canonical_markdown or "")
-        story = await create_story(
-            self._db,
-            user_id=user_id,
-            title=title,
-            stage=stage,
-            story_type=story_type,
-            summary=summary,
-            canonical_markdown=md,
-        )
-        await self._db.flush()
-        apply_infer_story_time_start_to_model(story)
-        if md.strip():
-            version = await create_story_version(
+        async with transactional(self._db):
+            story = await create_story(
                 self._db,
-                story_id=story.id,
-                version_no=1,
-                markdown_snapshot=md,
-                actor_type="ai",
-                source_type="generate",
+                user_id=user_id,
+                title=title,
+                stage=stage,
+                story_type=story_type,
+                summary=summary,
+                canonical_markdown=md,
             )
             await self._db.flush()
-            story.current_version_id = version.id
-            await _extract_and_store_image_intent(
-                self._db,
-                story=story,
-                version=version,
-                markdown=md,
-            )
-        if md.strip():
-            await memoir_repo.mark_chapters_dirty_for_story(self._db, story.id)
-        await self._db.commit()
+            apply_infer_story_time_start_to_model(story)
+            if md.strip():
+                version = await create_story_version(
+                    self._db,
+                    story_id=story.id,
+                    version_no=1,
+                    markdown_snapshot=md,
+                    actor_type="ai",
+                    source_type="generate",
+                )
+                await self._db.flush()
+                story.current_version_id = version.id
+                await _extract_and_store_image_intent(
+                    self._db,
+                    story=story,
+                    version=version,
+                    markdown=md,
+                )
+            if md.strip():
+                await memoir_repo.mark_chapters_dirty_for_story(self._db, story.id)
+        story_id = story.id
         if md.strip():
             from app.features.memoir.repo import get_chapter_ids_linked_to_story
             from app.features.story.post_commit import enqueue_story_post_commit_effects
 
-            chapter_ids = set(await get_chapter_ids_linked_to_story(self._db, story.id))
+            chapter_ids = set(await get_chapter_ids_linked_to_story(self._db, story_id))
             pc = enqueue_story_post_commit_effects(
                 user_id=user_id,
-                story_ids={story.id},
+                story_ids={story_id},
                 chapter_ids=chapter_ids,
                 trigger_source="manual_api",
                 need_compaction=False,
@@ -176,7 +179,7 @@ class StoryService:
                 pc.enqueued_chapter_recompose_count,
                 pc.errors,
             )
-        return story.id
+        return story_id
 
     async def append_version(
         self,
@@ -191,32 +194,33 @@ class StoryService:
         """Append new version, update canonical_markdown, return version_id."""
         story = await get_story_by_id(self._db, story_id)
         if not story:
-            raise ValueError(f"Story {story_id} not found")
+            raise NotFoundError(f"Story {story_id} not found")
         md = strip_asset_image_refs_from_markdown(markdown_snapshot or "")
         parent_id = story.current_version_id
         version_no = (await count_story_versions(self._db, story_id)) + 1
-        version = await create_story_version(
-            self._db,
-            story_id=story_id,
-            version_no=version_no,
-            markdown_snapshot=md,
-            actor_type=actor_type,
-            source_type=source_type,
-            parent_version_id=parent_id,
-            prompt_meta=prompt_meta,
-        )
-        version.change_summary = change_summary
-        story.current_version_id = version.id
-        story.canonical_markdown = md
-        apply_infer_story_time_start_to_model(story)
-        await _extract_and_store_image_intent(
-            self._db,
-            story=story,
-            version=version,
-            markdown=md,
-        )
-        await memoir_repo.mark_chapters_dirty_for_story(self._db, story_id)
-        await self._db.commit()
+        async with transactional(self._db):
+            version = await create_story_version(
+                self._db,
+                story_id=story_id,
+                version_no=version_no,
+                markdown_snapshot=md,
+                actor_type=actor_type,
+                source_type=source_type,
+                parent_version_id=parent_id,
+                prompt_meta=prompt_meta,
+            )
+            version.change_summary = change_summary
+            story.current_version_id = version.id
+            story.canonical_markdown = md
+            apply_infer_story_time_start_to_model(story)
+            await _extract_and_store_image_intent(
+                self._db,
+                story=story,
+                version=version,
+                markdown=md,
+            )
+            await memoir_repo.mark_chapters_dirty_for_story(self._db, story_id)
+        version_id = version.id
         from app.features.memoir.repo import get_chapter_ids_linked_to_story
         from app.features.story.post_commit import enqueue_story_post_commit_effects
 
@@ -236,7 +240,7 @@ class StoryService:
             pc.enqueued_chapter_recompose_count,
             pc.errors,
         )
-        return version.id
+        return version_id
 
     async def link_evidence(
         self,
@@ -248,15 +252,15 @@ class StoryService:
         weight: float | None = None,
     ) -> None:
         """Add evidence link. Caller must ensure story exists."""
-        await create_story_evidence_link(
-            self._db,
-            story_id=story_id,
-            evidence_type=evidence_type,
-            evidence_id=evidence_id,
-            role=role,
-            weight=weight,
-        )
-        await self._db.commit()
+        async with transactional(self._db):
+            await create_story_evidence_link(
+                self._db,
+                story_id=story_id,
+                evidence_type=evidence_type,
+                evidence_id=evidence_id,
+                role=role,
+                weight=weight,
+            )
 
     async def get_stories(
         self, user_id: str, *, status: str | None = "active"