Refactor media source handling and update collection options

- Updated `IMediaSourceCatalog` to support user-added media sources, allowing dynamic editing and management of sources.
- Removed the `UrlListSource` class as its functionality is now integrated into the new catalog structure.
- Enhanced `CollectOptions` to default `RequireProxy` to true, ensuring stricter handling of proxy requirements.
- Improved error handling in `ParseError` to include a `Subject` field for better context on failures.
- Adjusted dependency injection to reflect changes in media source management, removing old source registrations.
- Introduced background proxy checks to ensure a more robust proxy pool management during collection processes.

These changes streamline the media collection process and improve the overall user experience by providing clearer error reporting and more flexible source management.

test-service/README.md

@@ -0,0 +1,66 @@
+# AvParser image test service
+
+A tiny, dependency-free web service that serves a deterministic image for any id matching a
+configured pattern. It exists as a **target** for the app's pattern sources: point a source at one of
+its endpoints and let the collector generate ids.
+
+Each valid id returns a unique identicon PNG (derived from `SHA-256(id)`), so fetching the same id
+twice yields the same bytes — which is what makes the collector's duplicate detection observable.
+
+## Endpoints
+
+Endpoints are declared in [`appsettings.json`](appsettings.json) under `ImgTest:Endpoints`. The
+defaults:
+
+| Path            | Id length | Characters        | Extension |
+| --------------- | --------- | ----------------- | --------- |
+| `/test1/{id}`   | 6–8       | letters + digits  | `.jpg`    |
+| `/test2/{id}`   | 8–12      | digits only       | none      |
+| `/test3/{id}`   | 8         | hex (`0-9a-f`)    | `.png`    |
+
+Anything not matching a pattern (`/test1/xx`, `/test2/abcd1234`) returns `404`. `GET /` lists the
+endpoints; `GET /healthz` returns `ok`.
+
+Example: `GET /test1/2dfhyuj.jpg` → `200 image/png`.
+
+### Adding an endpoint
+
+Add an object to `ImgTest:Endpoints`:
+
+```json
+{ "Path": "test4", "MinLength": 4, "MaxLength": 6, "Alphabet": "Custom", "Chars": "abcdef012", "Extensions": [".png"] }
+```
+
+`Alphabet` is one of `LettersLower`, `LettersUpper`, `Letters`, `Digits`, `Alphanumeric`, `HexLower`,
+or `Custom` (which uses `Chars`).
+
+## Miss rate
+
+Set `ImgTest:MissRate` (0..1) to make a deterministic fraction of otherwise-valid ids answer `404`,
+emulating a site where most guessed ids do not exist. It is stable per id, so a run is repeatable.
+Override at deploy time with the `ImgTest__MissRate` environment variable.
+
+## Run
+
+```bash
+docker compose up --build
+```
+
+The container listens on `http://localhost:8080`. It speaks plain HTTP — **TLS for a public domain
+such as `https://imgtest.example` is terminated by your own reverse proxy**, which forwards to this
+container. A Caddy example:
+
+```
+imgtest.example {
+    reverse_proxy localhost:8080
+}
+```
+
+Or run it directly without Docker:
+
+```bash
+dotnet run
+```
+
+> This service is intentionally **not** part of `AvParser.slnx` and is not built by `./build.ps1`.
+> It is a standalone support tool with its own Docker context.