Support explicit MIME type overrides

## Motivation

Shadow currently derives `Content-Type` from content signatures and the managed path extension. Some domain-specific or uncommon formats need an explicit media type that cannot be inferred reliably.

## Proposed direction

Add backend-neutral configuration for extension-based MIME overrides, for example:

```toml
[http.content_types]
wasm = "application/wasm"
glb = "model/gltf-binary"
custom = "application/x-custom"
```

Suggested precedence:

1. explicit configured override
2. specific content signature
3. managed path extension
4. `application/octet-stream`

The resolved `Content-Type` should be recomputed by `publish` and `check --remote`. It is intentionally not stored in the ref; changes to MIME inference are treated as Shadow behavior changes. This issue does not include `Content-Disposition` handling.

Start by locating configuration handling and MIME resolution used by publish and check --remote. Trace how content signatures and managed path extensions currently determine Content-Type, then verify the configured-over-signature-over-extension-over-default precedence and that the resolved type is recomputed without being stored in the ref.

Written by the indexing model from the issue text.