# Task 01 — Image toggle: BOR_IMAGES + extensions + dir, off by default

**Phase:** `122_image_documents` · **Source:** `TODO.md:6` — "The user should be able to turn on and off image support in their .env depending on whether their model supports it."

## Objective
The single env knob for image support exists and is surfaced — `BOR_IMAGES` (default **false**), `BOR_IMAGE_EXTENSIONS`, `BOR_IMAGE_DIR` — with `GET /api/config` exposing the flag for UI gating. Toggle off = byte-identical behavior to pre-phase.

## Work
1. `app/config.py` — three new `Settings` fields (house docstring style, the `upload_dir`/`llm_retries` precedents):
   - `images: bool = False` — `BOR_IMAGES`, `0`/`false` off (LOCKED A3 default). Docstring: master switch for image-document indexing (phase 122) — off by default, enable only when the chat model supports vision (descriptions come from it).
   - `image_extensions: str = "png,jpg,jpeg,webp,gif,bmp"` — `BOR_IMAGE_EXTENSIONS`, comma-separated, case-insensitive; a property/parse into a lowercased-dotted frozenset (the `import_extension_set` precedent) — the image set is SEPARATE from `import_extension_set` (images are never user-added via `BOR_IMPORT_EXTENSIONS`).
   - `image_dir: str = "~/bor-sources/images"` — `BOR_IMAGE_DIR`, raw string, `Path.expanduser()` applied by the importer (the `sources_dir`/`upload_dir` convention) — the persistent home for image bytes (uploads are replaced, checkouts re-cloned — the copy must outlive the source file).
2. `.env.example` — the three entries with the comment block: off by default + the vision-model dependency note (LOCKED A3).
3. `app/api/config.py:30` — the `app_config` dict gains `"images": settings.images` (the dict is `str | bool`-valued — bools already allowed). Extend the docstring: consumed by the chat composer (phase 123) to show/hide the attach control, optionally by the Sources page.
4. ASSUMPTION: `GET /api/config` is already anonymous-readable (the UI gates on it pre-login in phase 123 — no auth change here).

## Testing & Quality
- Unit: `tests/unit/test_image_documents.py` (task 06 finalizes) — defaults (`images` False, extensions frozenset `{".png", …}` with the dotted form the matchers expect, dir default), env overrides, the frozenset parse is case-insensitive and trims spaces.
- Integration: the existing `GET /api/config` test asserts the new `images` key (default false in the test env).
- Coverage: **>90%** on the touched modules.

## Completion Criteria
- [ ] `Settings()` with no env: `images is False`, the extension set is the six defaults, `image_dir` is the default path.
- [ ] `GET /api/config` returns `images: false` in the default test env (byte-check the other keys unchanged).
- [ ] `uv run pytest` green; `uv run ruff check . && uv run pyright` clean.