docs: README + DEVELOP — code map, Settings, bucket routing, troubleshooting

- README: refresh "what it does", "architecture at a glance", "containers"
  table, "configuration", "storage layout" (with usage_type→bucket
  routing table), and add "documentation" section pointing to
  ARCHITECTURE / HANDOVER / DEVELOP / CLAUDE.

- DEVELOP: add the developer-facing guide that was previously only in
  CLAUDE.md. Covers code layout, the Settings singleton, conventions
  (DB / Storage / Auth / Permission gates / Outbox / async-sync
  signatures), local dev workflow, common tasks (adding a DAG endpoint,
  env var, MySQL table, RustFS bucket, schedule node type), tests
  status, and a troubleshooting section with the four real bugs hit
  this session (greenlet, MySQL, Jupyter 401, Schedule not advancing).

DEVELOP.md

@@ -286,16 +286,37 @@ See "Adding a new env var" above.
 
 ### Wire a new RustFS bucket
 
+The current three buckets are wired in `backend/storage_api.py:resolve_bucket`:
+
+```python
+BUCKET_FOR_USAGE: dict[str, str] = {
+    "working_copy":     settings.rustfs_workspace_bucket,
+    "public_script":    settings.rustfs_workspace_bucket,
+    "data_resource":    settings.rustfs_workspace_bucket,
+    "snapshot":         settings.rustfs_workspace_bucket,
+    "version_artifact": settings.rustfs_version_bucket,
+    "run_log":          settings.rustfs_run_log_bucket,
+    "run_result":       settings.rustfs_run_log_bucket,
+}
+```
+
+To add a fourth bucket:
+
 1. Add the env var to `Settings`:
    ```python
    rustfs_<feature>_bucket: str = Field(default="<feature>", description="...")
    ```
-2. Add to `.env.example`.
-3. Extend the `Literal` in `ServerObjectRequest.usage_type` to include
-   the new `usage_type` value, if applicable.
-4. In the consumer of the bucket, branch on `usage_type` (or the
-   `bucket_name` argument) and pick the right bucket via
-   `settings.rustfs_<feature>_bucket`.
+2. Add to `.env.example` with a one-line comment.
+3. Extend the `Literal` in `common/storage/schemas.py` (in
+   `ServerObjectRequest.usage_type`, `CreateUploadRequest.usage_type`,
+   `CompleteUploadRequest.usage_type`) to include the new value.
+4. Add an entry in `BUCKET_FOR_USAGE` mapping the new `usage_type` to
+   the new bucket env var.
+5. Add the bucket to the `ensure_bucket` loop in
+   `backend/main.py` lifespan.
+
+A workspace's `artifact_bucket` column (when non-null) overrides the
+default for that workspace, regardless of `usage_type`.
 
 ### Add a new schedule node type