feat(sse): per-client live subscription filter (#86)

* feat(sse): per-client live subscription filter

Lets a connected client narrow its SSE stream to a single session
without forcing an EventSource reconnect. With many open sessions
(N tabs in the UI, all generating output), this cuts terminal-event
SSE traffic by roughly Nx — we only send the actively-rendered
session's bytes instead of all of them.

The existing ?sessions= query filter only worked at connect time;
narrowing or widening it required tearing down the EventSource and
losing in-flight messages. That was acceptable when filters were set
once at page load, but the UI now flips active sessions on every
tab switch.

How it works
============

- Client generates a stable per-page UUID (`_clientId`) once at
  CodemanApp construction and includes it on the SSE URL:
    GET /api/events?clientId=<uuid>&sessions=<active-id>
- Server records a `clientId -> reply` mapping in addition to the
  existing `reply -> sessionFilter` map.
- New endpoint:
    POST /api/events/subscribe { clientId, sessions: string[] | null }
  updates the in-memory filter for the matching reply. 204 on success,
  404 if the client isn't known yet (race on first selectSession after
  reconnect — the next reconnect carries the filter via the URL).
- On every selectSession the client fires a fire-and-forget POST. No
  reconnect, no re-init, no replay buffer needed.

Behavioural change to broadcast()
=================================

The per-event session filter is removed from `broadcast()`. Previously
that path filtered lifecycle/metadata events (`session:created`,
`session:updated`, `ralph:*`, `hook:*`) by extracting a `sessionId` from
the payload. With per-client narrow filters, that meant a client
subscribed to session A would never see session:created for B and the
sidebar would silently de-sync.

The new contract:
- **Lifecycle/metadata events** (low-volume, UI-correctness critical)
  broadcast to all clients regardless of filter.
- **Terminal events** (high-volume, the actual reason for filtering)
  apply the filter in `flushSessionTerminalBatch` (already there;
  unchanged).

`extractSessionId()` was only used by the old broadcast() filter and
has been removed.

Files
=====

- src/web/sse-stream-manager.ts (+34/-29): add `sseClientsById`,
  optional `clientId` arg to addClient/removeClient cleanup, new
  `updateClientFilter()`, and the broadcast() change above.
- src/web/server.ts (+22/-3): parse `clientId` on /api/events, pass
  to `addClient`, register POST /api/events/subscribe handler.
- src/web/public/app.js (+41/-1): generate `_clientId`, build the
  EventSource URL with both clientId + active session, add
  `_updateSseSubscription()`, call it on selectSession.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* test(sse): update operation-lightspeed to match broadcast-all contract

Lifecycle events (session:*, case:*) now reach every connected SSE
client; only session:terminal is gated by the per-client filter.
Updates the four assertions in operation-lightspeed.test.ts that
encoded the old "filter applies to all events" contract.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: arkon <arkon.85@hotmail.com>

src/web/server.ts

@@ -579,9 +579,11 @@ export class WebServer extends EventEmitter {
       }
 
       // Parse optional session subscription filter from query parameter.
-      // /api/events?sessions=id1,id2 — client only receives events for those sessions.
-      // /api/events (no param) — client receives all events (backwards-compatible).
-      const query = req.query as { sessions?: string };
+      // /api/events?sessions=id1,id2 — client only receives session:terminal
+      //   events for those sessions (other events broadcast to all clients).
+      // /api/events?clientId=<uuid> — enables live filter updates via
+      //   POST /api/events/subscribe without reconnecting.
+      const query = req.query as { sessions?: string; clientId?: string };
       let sessionFilter: Set<string> | null = null;
       if (query.sessions) {
         const ids = query.sessions
@@ -592,6 +594,7 @@ export class WebServer extends EventEmitter {
           sessionFilter = new Set(ids);
         }
       }
+      const clientId = typeof query.clientId === 'string' && query.clientId ? query.clientId : undefined;
 
       reply.raw.writeHead(200, {
         'Content-Type': 'text/event-stream',
@@ -603,7 +606,7 @@ export class WebServer extends EventEmitter {
       // Track tunnel clients — cloudflared proxies locally so req.ip is always
       // 127.0.0.1; detect tunnel traffic via Cf-Connecting-Ip header instead.
       const isRemote = !!req.headers['cf-connecting-ip'];
-      this.sse.addClient(reply, sessionFilter, isRemote);
+      this.sse.addClient(reply, sessionFilter, isRemote, clientId);
 
       // Send initial state
       // Use light state for SSE init to avoid sending 2MB+ terminal buffers
@@ -618,6 +621,20 @@ export class WebServer extends EventEmitter {
       });
     });
 
+    // Live subscription update — change a connected client's session filter
+    // without forcing an SSE reconnect. Body: { clientId, sessions: string[] | null }
+    // Empty/null sessions array = remove filter (receive all session:terminal events).
+    this.app.post('/api/events/subscribe', (req, reply) => {
+      const body = (req.body || {}) as { clientId?: string; sessions?: string[] | null };
+      if (!body.clientId || typeof body.clientId !== 'string') {
+        reply.code(400).send({ error: 'clientId required' });
+        return;
+      }
+      const sessions = Array.isArray(body.sessions) ? body.sessions.filter((s) => typeof s === 'string') : null;
+      const updated = this.sse.updateClientFilter(body.clientId, sessions);
+      reply.code(updated ? 204 : 404).send();
+    });
+
     // Global error handler for structured errors thrown by findSessionOrFail
     this.app.setErrorHandler((error, _req, reply) => {
       const statusCode = (error as { statusCode?: number }).statusCode ?? 500;