feat(net): trusted-proxy depth so real client IPs survive a proxy — #2453

Fixes the defect the operator spotted in #370 immediately after it shipped:
auth.ClientIP ignored X-Forwarded-For whenever RemoteAddr was public, so a
proxy on a public address — a separate host, or a CDN, i.e. anyone running
this publicly, since public means TLS means a proxy — recorded the PROXY for
every session. created_ip and last_ip were then always equal and the
"Address changed" signal could never fire. The feature looked like it worked
and reported nothing.

Replaced with the standard trusted-hop model (Rails, Caddy, Traefik, nginx).
XFF grows left-to-right as each proxy appends the peer it received from, so
for client -> CDN -> own-proxy -> app the app sees [client, CDN] with
RemoteAddr = own-proxy, and the client sits at XFF[len - hops]:

  0  RemoteAddr, XFF ignored — no proxy
  1  the address your own proxy observed
  2  through a CDN in front of your proxy

Default 1, per the operator: publicly reachable means a TLS terminator in
front.

The cost is real and stated rather than hidden. hops >= 1 DECLARES that a
proxy exists; set it with no proxy, or deeper than the actual chain, and the
index reaches attacker-supplied entries, letting a visitor choose which
address their own session shows — defeating exactly the detection #370 is
for. That's inherent to the model, which is why 0 is a first-class value and
the admin card says "count your proxies, don't guess high" instead of just
exposing a number. Both mis-set shapes are pinned by tests so they stay known
consequences rather than surprises.

Migration 0053 + internal/netsettings, cached under an RWMutex. That's not an
optimisation: ClientIP runs in RequireUser for every authenticated request, so
a per-request query would put the database on the critical path of the whole
API. New() always returns a usable service so a boot-time DB hiccup degrades
to the default instead of breaking that path (rule #131), and Hops() is
nil-safe because test routers construct middleware without it.

RequireUser now takes a func() int rather than an int — the value is
operator-editable at runtime while the middleware is built once at boot, and
reading it per request is what makes a save take effect with no restart
(rule #25).

The admin card is verifiable, not just configurable: it reports the address
the CURRENT setting resolves THIS request to, the raw forwarded chain, and the
socket peer — so you set the number, save, and confirm the address matches the
machine you're on. It also counts the arriving chain and says how many proxies
that implies. GET/PUT both return that payload, PUT recomputed under the new
value, so the effect is visible without a reload.

Also fixes styling in the #370 card that CI could not catch: text-destructive
and bg-destructive don't exist in this Tailwind config — the palette is
colors.action.destructive — so the "Address changed" warning and the
sign-out-others button were rendering unstyled. Both now use
text-action-destructive / bg-action-destructive / text-action-fg.

Not done here: requestlog.go still logs raw RemoteAddr and will disagree with
the sessions UI about who connected. Left for its own change.

internal/auth/session.go

@@ -56,7 +56,13 @@ const SessionCookieName = "minstrel_session"
 // bearer header and puts the dbq.User in request context via userCtxKey.
 // Requests without a valid session return 401 with no body so callers don't
 // leak whether the username existed (matches the /rest/* auth posture).
-func RequireUser(pool *pgxpool.Pool) func(http.Handler) http.Handler {
+//
+// trustedHops supplies the reverse-proxy depth used to record the session's
+// current address (#2453). It's a func rather than an int because the value
+// is operator-editable at runtime and this middleware is constructed once at
+// boot — reading it per request is what makes an admin change take effect
+// without a restart. Passing nil means "trust nothing", i.e. the socket peer.
+func RequireUser(pool *pgxpool.Pool, trustedHops func() int) func(http.Handler) http.Handler {
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			token := sessionTokenFromRequest(r)
@@ -103,7 +109,7 @@ func RequireUser(pool *pgxpool.Pool) func(http.Handler) http.Handler {
 			// surface exists to show, and it costs nothing extra here.
 			if err := q.TouchSessionLastSeen(r.Context(), dbq.TouchSessionLastSeenParams{
 				ID:     sess.ID,
-				LastIp: ClientIP(r),
+				LastIp: ClientIP(r, hopsOf(trustedHops)),
 			}); err != nil {
 				slog.Warn("api: touch session last_seen failed", "err", err)
 			}