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/types/mesh.ts

@@ -28,6 +28,17 @@ export interface MeshAlias {
 
 export type MeshReachableMode = 'local' | 'pilot' | 'proxy' | 'unreachable';
 
+/**
+ * State of the peer→central reverse path for a proxy-mode peer. Central
+ * maintains a persistent forward WS to every mesh-enabled proxy peer;
+ * peer→central traffic multiplexes over that same bridge. The four values:
+ *   - `connected`: bridge open, reverse multiplex available.
+ *   - `connecting`: dial in flight (transient).
+ *   - `unavailable`: bridge not open, no dial in flight; central will redial on its next reconcile tick.
+ *   - `not_applicable`: local node, pilot-mode peer, or mesh disabled.
+ */
+export type MeshReverseCallbackStatus = 'connected' | 'connecting' | 'unavailable' | 'not_applicable';
+
 export interface MeshNodeStatus {
     nodeId: number;
     nodeName: string;
@@ -40,6 +51,8 @@ export interface MeshNodeStatus {
     reachableMode: MeshReachableMode;
     /** Operator-facing reason when `reachableMode === 'unreachable'`. Null otherwise. */
     reachableReason: string | null;
+    /** Peer→central reverse path state. `not_applicable` for non-proxy peers. */
+    reverseCallbackStatus: MeshReverseCallbackStatus;
     optedInStacks: string[];
     activeStreamCount: number;
 }