R4: Add Loopia DNS-01 provider to the ACME provider factory

[mhenrixon]

lego supports a Loopia DNS-01 provider but our acme provider factory (internal/server/acme/providers/factory.go) does not include it. wm3 parity needs it: the current Traefik tier issues *.wm3.se via a letsencrypt-dns-loopia resolver. Add loopia to the factory (env creds per lego: LOOPIA_API_USER / LOOPIA_API_PASSWORD), mirror an existing provider case, add the factory unit test case, document in README provider table. Discovered during the dynamic-domains investigation (#21).

[mhenrixon]

Moved out of the R4 milestone and into the new DNS epic: #77.

Rescoped slightly — this now lands after the registry refactor (#72), so it is one table entry rather than the five-site pattern the original description assumed ("mirror an existing provider case"). Credentials confirmed against the pinned lego v4.30.1: required LOOPIA_API_USER + LOOPIA_API_PASSWORD, optional LOOPIA_API_URL, LOOPIA_TTL, LOOPIA_PROPAGATION_TIMEOUT, LOOPIA_POLLING_INTERVAL, LOOPIA_HTTP_TIMEOUT.

The README table edit the original description asks for is also superseded — #75 generates it from the registry.