Rename SUBSD_* env vars to SUBS_* to match upstream refactor

The upstream SpacesOps app refactored its SUBSD_* environment variables to
SUBS_* (verified against server.js in spacesops/spacesops:v1.0.0). Update the
package to match so the configured values reach the app:

- main.ts: inject SUBS_URI_VALUE / SUBS_RPC_USER / SUBS_RPC_PASSWORD.
- configurePlatform action + storeJson: rename store keys subsd*→subs* and the
  field labels/descriptions (SUBS URL / SUBS RPC User / SUBS RPC Password).
- README, instructions, release notes: SUBSD→SUBS.

Note: the image's /app/setup-spacesops-env.sh still exports SUBSD_* defaults
(partial upstream refactor) — see report; vars the package does not set (e.g.
SUBS_RPC_URL) currently have no in-image default.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

instructions.md

@@ -44,7 +44,7 @@ your LAN, and do **not** expose its admin routes to the public internet.
 - **Show Admin Credentials** — view the fixed admin login (with the warning
   above).
 - **Configure Platform** — optionally set the Nostr relay, theme mode, CoinGecko
-  pricing, and a SUBSD backend. Pricing and subname-purchase features stay off
+  pricing, and a SUBS backend. Pricing and subname-purchase features stay off
   until you configure them here. Saving restarts the service.
 
 ## Backups