fix(relay): distinguish transient tenant-bind failures from unmapped hosts (#5030)

[yippietheproducer]

Fixes #5030 (the relay-generated half).

Problem

The WS door and every HTTP bridge endpoint collapsed all tenant-bind
failures to 404 "relay: no community is configured for this host". A DB
hiccup during a reconnect therefore looked identical to a decommissioned
relay, and clients that classify 404 as terminal (buzz-acp:
WebSocket(Http): 404 → terminal) gave up on healthy relays. #5030
measured a 22% reconnect-failure rate on hosted communities with agent
sessions going dark once the 5-attempt budget was exhausted.

Fix

bind_community now maps BindError to a status via
BindError::http_status():

• UnmappedHost → 404 (permanent — clients correctly stop retrying)
• Lookup → 503 (transient — clients retry 5xx; buzz-acp already
  classifies 429 | 502 | 503 | 504 as transient)

The rejection body stays byte-identical for both variants — never echo
the host, never leak the underlying error — preserving the anti-enumeration
property in the steady state. The only new signal is the status code, and
it is observable solely while the backend lookup is actually failing (the
narrow, documented tradeoff; the alternative made healthy relays
indistinguishable from decommissioned ones to every client).

Applied at every bind site: WS door (router.rs), audio handler, and
the four HTTP bridge endpoints (submit_event, query_events,
count_events, workflow).

Notes

• The other relay-visible error class in Relay: reconnects fail ~22% of the time on hosted communities (500 in auth path, 404 on WS upgrade) #5030 — error: internal error checking restriction state on the auth path — is the ban-check failing
  closed on a DB error, which is correct fail-closed behavior, and
  buzz-acp already treats error:-prefixed auth failures as transient
  (is_terminal_auth_failure). No change needed there.
• The 15–17 min connection recycling is client-handled per the reporter.

Verification

• cargo check -p buzz-relay — clean, zero warnings
• New tenant unit test asserting the status split (404 vs 503) and that
  both variants share an identical, error-free body
• Full buzz-relay lib suite: 848 passed / 1 failed — the failure
  (api::mesh_demo::demo_join_forwarded_arm_round_trips_echo) is a
  pre-existing timing flake, reproduced on the clean tree without this
  change