Add git-like file commit API (#15978)

### What problem does this PR solve?

| # | Method | Endpoint | Description | Git Equivalent |
|---|--------|----------|-------------|----------------|
| 1 | `POST` | `/api/v1/{prefix}/{folder_id}/commits` | Create a
snapshot commit with file changes (add/modify/delete/rename) | `git add`
+ `git commit` |
| 2 | `GET` | `/api/v1/{prefix}/{folder_id}/commits` | List commit
history (paginated) | `git log` |
| 3 | `GET` | `/api/v1/{prefix}/{folder_id}/commits/{commit_id}` | Get
commit detail with file changes | `git show` |
| 4 | `GET` | `/api/v1/{prefix}/{folder_id}/commits/{commit_id}/files` |
List file changes in a commit | `git show --name-status` |
| 5 | `GET` |
`/api/v1/{prefix}/{folder_id}/commits/diff?from=...&to=...` | Compare
two commits and return differences | `git diff` |
| 6 | `GET` | `/api/v1/{prefix}/{folder_id}/changes` | Get uncommitted
changes (add/modify/delete) | `git status` |
| 7 | `GET` | `/api/v1/{prefix}/{folder_id}/commits/{commit_id}/tree` |
Get the folder tree snapshot at commit time | `git ls-tree` |
| 8 | `GET` |
`/api/v1/{prefix}/{folder_id}/commits/{commit_id}/files/{file_id}/content`
| Get a file's content as it existed in a specific commit | `git show
HEAD:file` |
| 9 | `GET` | `/api/v1/{prefix}/{file_id}/versions` | Get version
history for a specific file across all commits | `git log -- file` |

Where `{prefix}/{id}` can be:
- `folders/{folder_id}` — direct folder access
- `workspaces/{workspace_id}` — alias of `folders/{folder_id}`
- `datasets/{dataset_id}` — resolves to the dataset's folder
- `memories/{memory_id}` — resolves to the memory's folder
- `skills/{skill_id}` — resolves to the skill's folder

### Type of change

- [x] New Feature (non-breaking change which adds functionality)
- [x] Documentation Update

api/db/db_models.py

@@ -956,6 +956,38 @@ class File2Document(DataBaseModel):
         db_table = "file2document"
 
 
+class FileCommit(DataBaseModel):
+    id = CharField(max_length=32, primary_key=True)
+    folder_id = CharField(max_length=32, null=False, help_text="workspace folder id", index=True)
+    parent_id = CharField(max_length=32, null=True, help_text="parent commit id", index=True)
+    message = CharField(max_length=512, default="", help_text="commit message")
+    author_id = CharField(max_length=32, null=False, help_text="user who created the commit", index=True)
+    file_count = IntegerField(default=0, help_text="number of files in this commit")
+    tree_state = LongTextField(null=True, help_text="JSON snapshot of the full folder tree at this commit")
+
+    class Meta:
+        db_table = "file_commit"
+
+
+class FileCommitItem(DataBaseModel):
+    id = CharField(max_length=32, primary_key=True)
+    commit_id = CharField(max_length=32, null=False, help_text="commit id", index=True)
+    file_id = CharField(max_length=32, null=False, help_text="file id", index=True)
+    operation = CharField(max_length=16, null=False, help_text="add / modify / delete / rename", index=True)
+    old_hash = CharField(max_length=64, null=True, help_text="old content hash", index=True)
+    new_hash = CharField(max_length=64, null=True, help_text="new content hash", index=True)
+    old_location = CharField(max_length=255, null=True, help_text="old storage location")
+    new_location = CharField(max_length=255, null=True, help_text="new storage location")
+    old_name = CharField(max_length=255, null=True, help_text="old file name (for rename)")
+    new_name = CharField(max_length=255, null=True, help_text="new file name (for rename)")
+
+    class Meta:
+        db_table = "file_commit_item"
+        indexes = (
+            (("commit_id", "file_id"), True),  # unique composite index
+        )
+
+
 class Task(DataBaseModel):
     id = CharField(max_length=32, primary_key=True)
     doc_id = CharField(max_length=32, null=False, index=True)