fix(mesh): route peer→central traffic over the existing forward WS (#1094)

* fix(mesh): route peer→central traffic over the existing forward WS

The reverse mesh callback path (`/api/mesh/proxy-tunnel-from-peer`) needed
SENCHO_PRIMARY_URL on central plus a publicly reachable origin from the
peer's perspective. In a typical homelab where central sits behind NAT,
peer→central dispatch silently failed at the dialer's short-circuit and
the headline "call any service on any node by hostname" worked one way
only.

The forward WS at `/api/mesh/proxy-tunnel` is already bidirectional end
to end. Make the bridge a persistent control-plane primitive: dial every
mesh-enabled proxy peer at startup, reconcile every 60 s, never idle-close.
Peer→central traffic multiplexes over the same WS via `tcp_open_reverse`.

Removed:
- `meshProxyTunnelFromPeer.ts` WS handler and dispatch
- `MeshCentralRegistry`, `PeerToCentralMeshSessionDialer`
- `mesh_handshake` first-frame state machine in `meshProxyTunnel.ts`
- `maybeSendBootstrap`, `buildHandshakeFrame` in the dialer
- `mesh_proxy_callback_bootstrap` capability and `maybeWarnUnsetPrimaryUrl`
- `mesh_centrals` table (drop migration; greenfield, no users)
- `PilotTunnelManager.replaceOrRegisterProxyBridge` (dead after handler removal)
- twelve associated unit/integration tests plus the peer-recovery branch
  in `MeshService.openCrossNode`

Added:
- `MeshService.proactiveBridgeFanout` selects every mesh-enabled proxy
  peer (no longer gated on `mesh_stacks` rows)
- `startBridgeReconcileLoop` runs the fanout every 60 s (override via
  `SENCHO_MESH_RECONCILE_INTERVAL_MS`)
- `MeshProxyTunnelDialer` default idle TTL is now `0` and exposes
  `isDialing(nodeId)` for the status surface
- `MeshNodeStatus.reverseCallbackStatus` discriminator
  (`connected | connecting | unavailable | not_applicable`) surfaced via
  `/api/mesh/status` and rendered as a pill in the Routing tab
- `openCrossNode` error message distinguishes "no proxy target" from
  "waiting for central to dial the reverse bridge"
- New tests: `mesh-service-proxy-tunnel-reconcile`,
  `mesh-status-reverse-callback`, `mesh-proxy-tunnel-dialer-no-idle-close`

SENCHO_PRIMARY_URL is no longer required for any mesh function.

* fix(mesh): rewrite proxy-tunnel reconcile test contents

The previous commit renamed the file but the rewritten test bodies stayed
unstaged on top of the rename. This commit lands the actual rewrite: the
fanout assertion now requires every mesh-enabled proxy peer to be dialed,
not just those with `mesh_stacks` rows, and adds a reconcile-tick
repeated-call test.

frontend/src/components/fleet/MeshStackTopologySheet.test.tsx

@@ -21,6 +21,7 @@ function makeNode(over: Partial<MeshNodeStatus> & Pick<MeshNodeStatus, 'nodeId'
         pilotConnected: over.pilotConnected ?? false,
         reachableMode: over.reachableMode ?? 'unreachable',
         reachableReason: over.reachableReason ?? null,
+        reverseCallbackStatus: over.reverseCallbackStatus ?? 'not_applicable',
         optedInStacks: over.optedInStacks ?? [],
         activeStreamCount: over.activeStreamCount ?? 0,
     };

frontend/src/components/fleet/RoutingNodeCard.tsx

@@ -72,6 +72,22 @@ export function RoutingNodeCard({
                                 unreachable
                             </span>
                         )}
+                        {status.reverseCallbackStatus === 'connecting' && (
+                            <span
+                                title="Central is dialing the reverse bridge to this peer."
+                                className="inline-flex items-center px-1.5 py-0.5 rounded-sm border border-card-border bg-card text-[10px] leading-3 font-mono uppercase tracking-[0.18em] text-stat-subtitle"
+                            >
+                                reconnecting
+                            </span>
+                        )}
+                        {status.reverseCallbackStatus === 'unavailable' && (
+                            <span
+                                title="Peer→central tunnel is between dials. Central will redial on its next reconcile tick."
+                                className="inline-flex items-center px-1.5 py-0.5 rounded-sm border border-amber-500/40 bg-amber-500/10 text-[10px] leading-3 font-mono uppercase tracking-[0.18em] text-amber-600 dark:text-amber-400"
+                            >
+                                reverse unavailable
+                            </span>
+                        )}
                     </div>
                     <div className="flex items-center gap-2">
                         <TogglePill