feat(nodepool): node groups, per-node caps, and local transcode fallback control (#126)

* feat(nodepool): node groups, per-node caps, and local transcode fallback control

Group co-located transcode and proxy nodes so transcoded streams are
served by a proxy on the same host/LAN instead of bouncing across the
internal network (fixes #93):

- New nodepool.Planner is the single selection entry point: it picks the
  transcode node and its group's proxy together (round-robin within the
  group), replacing the independent ProxyPool.Pick/TranscodePool.Acquire
  calls scattered across the native and jellycompat handlers, and absorbs
  the duplicated soft-affinity pick logic.
- A group is only eligible while all of its enabled members are healthy;
  ungrouped nodes keep the historical behavior.
- New per-node max_jobs cap (transcodes for transcode nodes, streams for
  proxies; NULL = unlimited), enforced via health-reported job counts
  plus short-lived reservations that expire once fresher health data
  arrives. Proxy health now reports real stream counts, including HLS
  sessions via idle-expiry tracking.
- New playback.local_transcode_fallback setting (default on) lets admins
  refuse API-server transcoding when no eligible node exists.
- Health checks now publish updated node copies under the pool lock
  instead of mutating shared structs in place, fixing a data race.
- Admin UI: group + cap fields on the node form, group/cap columns, and
  the new fallback toggle in playback settings.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* feat(nodepool): proxy bandwidth measurement and egress caps

Proxy nodes now measure their stream egress (rolling 60s average over
everything under /stream) and report it via the health endpoint. A new
per-proxy max_bandwidth_kbps cap lets the planner route new streams away
from saturated proxies:

- Admission combines the measured egress with the estimated bitrate of
  the new stream (transcode target bitrate, or source bitrate for direct
  play/remux) so a stream is only admitted where it fits.
- Recently admitted streams are bridged as bandwidth reservations for the
  meter window, since the rolling average only converges on a new
  stream's rate gradually.
- A group whose proxies lack bandwidth headroom is treated as full: its
  transcode nodes are skipped, same as the job cap.
- Admin UI: per-proxy "Max Egress Bandwidth (Mbps)" field and a live
  egress column; manual health checks return the measured rate.

Active streams are never interrupted - the cap only gates new admissions.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* perf(playback): trim node-mode time-to-stream-start

Distributed playback paid several avoidable costs before the first frame
that integrated mode doesn't have. This trims the safe ones:

- Web player preconnects to the stream origin (the proxy node) as soon as
  /playback/start returns, overlapping DNS/TCP/TLS handshakes with the
  transcode dispatch instead of paying them at the first manifest fetch.
- The transcode node no longer blocks its 202 on monitoring work: the
  Redis session-track write moves off the request path, and a replaced
  session's segment directory is renamed aside and deleted in the
  background instead of synchronously (RemoveAll of a long session can
  take seconds on slow disks during quality switches).
- The proxy's node-facing HTTP client gets a tuned transport: a larger
  idle-connection pool (Go's default of 2 per host causes connection
  churn and TLS re-handshakes when many viewers stream through one
  proxy->node pair) and a response-header timeout so a hung transcode
  node can no longer hang client requests indefinitely.
- jellycompat's remote transcode dispatch gains the same 10s timeout the
  native path has had; an unreachable node previously hung the compat
  manifest request until the OS gave up.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

web/src/api/types.ts

@@ -2938,6 +2938,10 @@ export interface StreamNode {
   enabled: boolean;
   healthy: boolean;
   active_jobs: number;
+  group: string | null;
+  max_jobs: number | null;
+  max_bandwidth_kbps: number | null;
+  egress_kbps: number;
   last_health_check: string | null;
   created_at: string;
 }
@@ -2946,17 +2950,25 @@ export interface CreateNodeRequest {
   name: string;
   type: string;
   url: string;
+  group?: string;
+  max_jobs?: number;
+  max_bandwidth_kbps?: number;
 }
 
 export interface UpdateNodeRequest {
   name?: string;
   url?: string;
   enabled?: boolean;
+  // Empty string clears the group; 0 clears the caps (unlimited).
+  group?: string;
+  max_jobs?: number;
+  max_bandwidth_kbps?: number;
 }
 
 export interface CheckNodeResponse {
   healthy: boolean;
   active_jobs: number;
+  egress_kbps: number;
 }
 
 // User-facing library (simplified, no admin fields)