refactor(matrix): migrate AS transaction receiver from apps/web to apps/api

[rozagerardo]

Context

The Matrix AS inbound transaction receiver (#2483) is being built in apps/web as a pragmatic first home. apps/api (the Fastify service) is currently not deployed anywhere — no Vercel project, no deploy workflow references it, only a local Dockerfile + docker-compose.yml. Its README describes it as the backend for the (paused) Hypha Mobile project.

Once mobile work resumes and apps/api is re-established as a deployed, always-on service, the receiver should move there: it is a long-lived webhook that Dendrite holds a retry queue against, which fits a persistent process better than serverless functions.

What this involves

• Add a Fastify route adapter over the existing shared receiver function. The core logic (verify → parse → dedupe → dispatch handoff) is already framework-agnostic from feat(matrix): inbound Application Service transaction receiver #2483, so this should be a thin adapter.
• Move the catch-up / reconciler cron equivalent to apps/api, or keep it calling the shared function.
• Re-point the Dendrite AS registration url: to the new host — Preview registration file first, then Prod — and restart Dendrite.
• Re-verify end-to-end: Dendrite reaches the endpoint, non-200 responses get retried, downtime catch-up still works.
• Remove the apps/web route once the new endpoint is confirmed live.

Depends on

• feat(matrix): inbound Application Service transaction receiver #2483 (receiver built in apps/web)
• Choosing a Hosting Provider for Mobile API Server (Fastify) #1666 (apps/api hosting-provider decision — apps/api is currently undeployed)
• apps/api deployment re-established (mobile work resumed — see hypha-dao/hypha-mobile#56)

Note

The code migration is small. The real work is the ops re-point + end-to-end re-verification — a careful cutover, Preview then Prod, roughly half a day. Do not estimate this as a rename.

[rozagerardo]

The apps/api deployment this depends on is tracked in #1666 ("Choosing a Hosting Provider for Mobile API Server (Fastify)"). apps/api is currently undeployed (local Docker only), so this migration is blocked until #1666 is resolved.