Feat: Serve images directly from Cloudflare R2 CDN

API responses now include file_url and thumbnail_url fields. When
S3_PUBLIC_BASE_URL is configured, these point to the CDN domain;
when unset, they fall back to the existing API proxy paths so local
dev requires no additional setup. UI updated to use response URL
fields directly instead of constructing proxy URLs client-side.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

specs/014-r2-cdn-serving/research.md

@@ -0,0 +1,51 @@
+# Research: CDN Image Serving
+
+## Decision 1: Where does URL construction logic live?
+
+**Decision**: In the image router's `_image_to_dict` helper, not in the `StorageBackend`.
+
+**Rationale**: The `StorageBackend` interface is responsible for put/get/delete of object bytes. Adding URL construction there conflates two concerns — storage operations and HTTP URL generation — and would require the storage abstraction to know about CDN configuration. The router already has access to application settings via `get_settings()` and knows the image ID and storage key, making it the natural place to construct URLs.
+
+**Alternatives considered**: Adding a `get_url(key)` method to `StorageBackend` — rejected because it leaks HTTP/CDN concerns into the storage abstraction, violating §2.3.
+
+---
+
+## Decision 2: Fallback URL format in local development
+
+**Decision**: Relative paths (`/api/v1/images/{id}/file`, `/api/v1/images/{id}/thumbnail`) when `S3_PUBLIC_BASE_URL` is not set.
+
+**Rationale**: Relative paths work regardless of the host the app is running on, require no additional configuration, and match how the UI currently constructs these URLs via `getFileUrl(id)` and `getThumbnailUrl(id)`. An absolute fallback would require `API_BASE_URL` to be set in local dev, adding unnecessary setup friction.
+
+**Alternatives considered**: Absolute URL fallback using `API_BASE_URL` — rejected because it adds a mandatory config dependency where none exists today.
+
+---
+
+## Decision 3: Trailing slash normalisation
+
+**Decision**: Strip trailing slash from `S3_PUBLIC_BASE_URL` at construction time using `rstrip('/')` in the config validator or at point of use.
+
+**Rationale**: Prevents double-slash URLs (`https://cdn.example.com//key`) if the operator includes a trailing slash in the configured value. Simple, defensive, zero-cost.
+
+---
+
+## Decision 4: Proxy endpoints retained or removed?
+
+**Decision**: Retained, fully functional, unchanged.
+
+**Rationale**: Spec FR-005 explicitly requires them. They serve as the local dev fallback and a safety net if the CDN is temporarily unavailable or misconfigured. Removing them would break local development immediately.
+
+---
+
+## Decision 5: `storage_key` and `thumbnail_key` in API response
+
+**Decision**: Keep both fields in the response alongside the new `file_url` and `thumbnail_url`.
+
+**Rationale**: Removing them is a breaking API change. The UI currently reads `thumbnail_key` to decide whether a thumbnail exists. After this change the UI will use `thumbnail_url` (null when no thumbnail), but the keys remain in the response for backward compatibility with any tooling.
+
+---
+
+## Decision 6: Settings access in `_image_to_dict`
+
+**Decision**: Pre-compute the CDN base URL string once per request at the endpoint level and pass it into `_image_to_dict` as a parameter, rather than calling `get_settings()` inside the helper.
+
+**Rationale**: Keeps `_image_to_dict` a pure function (easier to test), avoids calling `get_settings()` inside a helper that is called in a loop (image list endpoint), and makes the dependency explicit.