casework serve: OIDC discovery against a loopback http issuer fails without naming the reason

Found while writing the public tutorial `Review BReg changes in Casework`.

## What Happened

`casework serve` with `authentication.oidc.jwksSource` left at its discovery default, pointed at a Registry Mint on `http://127.0.0.1:`, failed at startup with:

```
the OIDC issuer could not be initialized
```

The message does not say whether the issuer was refused for its scheme, its host, or an unreachable discovery document. The working configuration is a static JWKS reference:

```yaml
jwksSource:
kind: static
documentRef: secret:file/mint-jwks
```

which is what `caseworkctl dev` writes.

## Expected Behavior

The startup failure names the reason (for example "discovery requires an https issuer; loopback http issuers need a static jwksSource"). Whether loopback http discovery should be allowed at all is a separate call; the message is the bug.

## Environment

Registry Stack 0.30.0 release binaries, macOS.

Triage: later.

Start by reproducing the failure with `casework serve` using an OIDC discovery default and a loopback `http://127.0.0.1:` issuer, then trace the startup OIDC initialization entry point. Compare it with the static `jwksSource` configuration written by `caseworkctl dev`. Done means the startup error identifies whether the issuer scheme, host, or discovery document caused the failure.

Written by the indexing model from the issue text.