fix: interop forward built a double slash, so the product never registered

[benjamsf]

Relates to the BattleLog proof-of-concept test. Do not merge without a maintainer's review — this touches every product integration call.

User-Facing Summary

• POST /api/v1/product/interop/{tgt} answered 200 while the target product never received the request, so a product could never be registered for interop with another. In a live deployment BattleLog stayed permanently unregistered with matrixrmapi: every /interop/authz returned 403, and the room list, invite acceptance and Space search all failed with it.
• Cause: views.py passed "/api/v1/interop/add" — a leading slash — and _method_to_product joins it as f"{productconf['api']}{url_suffix}". Every manifest api value ends in /, so the request went to https://matrix.<domain>:4626//api/v1/interop/add. yarl preserves the doubled slash and Starlette does not route it, so the product 404s.
• That 404 becomes an aiohttp.ClientError, which _method_to_product swallows into None, and add_interop maps None to HTTP 200 with success: false — so the caller is told nothing useful.
• This was the only call site in the codebase with a leading slash; descriptions, instructions, user CRUD and healthcheck all pass a relative suffix. That is exactly why interop alone failed while everything else worked over the same nginx, port and client certificate.

Why It's Good for the Product (Release Goal)

• Product-to-product interop works at all; without it any product needing credentials from a sibling is silently dead.
• The failure mode was the expensive kind: a success status hiding a total failure, three unrelated-looking symptoms in another product's UI, and nothing in the logs naming the cause.
• The join now normalises both sides, so a future caller cannot reintroduce it. Behaviour-preserving for the existing relative call sites.

Verified

• yarl 1.24.5 keeps the path as //api/v1/interop/add rather than collapsing it.
• A mounted FastAPI app answers 200 for /api/v1/interop/add and 404 for //api/v1/interop/add.
• Linters clean on both changed files. The full suite needs infrastructure I do not have locally, so CI is the check that matters here.