fix(operator): harden S3/S2/S4 per adversarial review

9 confirmed findings (review wbdf3c0sd), all fixed:

S4 (capabilities):
- REGRESSION: /capabilities reported grin_relay:false for a working relay (S4
  wrongly coupled it to Grin chain serviceability). The relay is a non-custodial
  mailbox — wallet broadcasts locally — so it is decoupled again. Removed the now-
  dead grin_relay downgrade note.

S3 (bootstrap latch):
- A DELETE of the latch row escaped Locked without Tampered detection on a
  CLI-bootstrapped-but-no-users instance (adoption keyed off `users` only). Boot
  now adopts on has_any_users() OR has_any_admin_keys(), so a deleted latch
  re-latches Locked instead of silently dropping to uninitialized.
- bootstrap_admin's single-bootstrap guard was a check-then-insert race (two
  concurrent `setup` could seed two admins). It now takes the admin-keys mutation
  advisory lock (shared with add/revoke/rotate) at the top of its tx.

S2 (admin posture + landing):
- Hard bind guard matched loopback by string prefix ("localhost".starts_with) —
  "localhost.evil.com" slipped past. Now resolves via to_socket_addrs and requires
  EVERY resolved addr to be loopback.
- host_allowed's IPv6 loopback check was an unanchored prefix ("[::1]" matched
  "[::1].evil"); now anchored to exactly "[::1]" / "[::1]:port". +regression tests.
- Anti-clickjacking headers (X-Frame-Options/CSP) are now applied on the 403
  Host-reject path too (computed on both branches, headers added once).
- GET /api/v1/server-info now returns a BARE 404 when landing is off (the
  {error,code} envelope itself hinted the route exists).
- Public landing `chains` now uses the SAME serviceability predicate as
  /capabilities (no over-advertising an unserviceable chain).
- Removed the dead/misleading PUBLIC_EXPOSE_FEATURES toggle (never read; the
  feature tuple is intentionally never emitted anonymously).

Dismissed after verification: latch chains-by-flag (dup), title length bound
(operator-owned + escaped), prices:true with empty PRICES_ASSETS (documented).
140 lib + 140 integration green, no openapi drift, scans clean.

src/api/capabilities.rs

@@ -63,7 +63,7 @@ pub struct CapabilitiesResponse {
 /// present). A chain whose flag is on but whose source is unconfigured reports
 /// `enabled:false` — so `/capabilities` never advertises a chain that 404s, and
 /// a missing-secret downgrade is indistinguishable from a deliberately-off chain.
-fn chain_serviceable(config: &Config, asset: &str) -> bool {
+pub(crate) fn chain_serviceable(config: &Config, asset: &str) -> bool {
     let f = &config.features.chains;
     let c = &config.chains;
     match asset {
@@ -106,8 +106,9 @@ pub fn effective_capabilities(config: &Config) -> CapabilitiesResponse {
             },
         },
         features: FeatureCapabilities {
-            // The relay needs Grin chain access to be serviceable.
-            grin_relay: config.features.grin_relay && chain_serviceable(config, "grin"),
+            // The relay is a non-custodial mailbox (the wallet broadcasts
+            // locally), so it is NOT coupled to this backend's Grin chain access.
+            grin_relay: config.features.grin_relay,
             prices: config.features.prices,
             // Nostr identity needs the canonical PUBLIC_API_URL.
             nostr_identity: config.features.nostr_identity