proxy: verify TLS identities for HTTPS backend services

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## Summary

The reverse-proxy transport builds a `RootCAs` pool from service certificates but also sets `tls.Config.InsecureSkipVerify = true`. This disables certificate-chain and hostname verification, so the configured root pool does not authenticate the backend.

## Impact

A network-positioned attacker can impersonate an HTTPS backend and observe or alter proxied requests, responses, authorization material, or configured upstream credentials.

## Compatibility considerations

Simply changing the flag to false may break deployments that connect by IP address, use private/self-signed certificates, or have a certificate name different from `Service.Address`. The migration should be explicit rather than silently preserving insecure behavior.

Possible approaches:

- Add an explicit backend TLS server name and perform normal chain/hostname verification.
- Support deliberate certificate/SPKI pinning for private certificates.
- Keep insecure verification only as a clearly named opt-in compatibility setting, with a warning.

## Acceptance criteria

- An untrusted or wrong-identity backend certificate is rejected.
- A configured private CA or pinned certificate is accepted.
- IP-address backends have a documented secure configuration path.
- Plain HTTP service behavior is unchanged.

Start by locating the reverse-proxy transport and inspect how its RootCAs pool and InsecureSkipVerify settings are populated for HTTPS services. Use the acceptance criteria to define done: reject untrusted or wrong-identity certificates, accept configured private CAs or pins, document secure IP-address configuration, and preserve plain HTTP behavior.

Written by the indexing model from the issue text.