R-344: restore the idle-connection timeout our hand-rolled transports lost

Every client here pins TLS, so none can use http.DefaultTransport and each
hand-rolls its own. A composite literal takes IdleConnTimeout ZERO, which
means retain idle connections forever -- not "use a sane default".
pbsTargetsFromPVE builds a fresh pbs.Client every cycle and drops the
previous one, and an abandoned http.Transport does not close its
connections. One stranded socket per cycle, on both sides, forever.

Measured: 388 established connections on ep0 over 46 h, 194 per box, zero
closed in a 31-minute window. pvestatd and proxmox-backup-client made
162,404 requests in the same window and leaked none.

New leaf package internal/httpx owns the default (90s, http.DefaultTransport's
own value) and NewTransport, which returns a FRESH transport per call and
treats <=0 as "use the default", never "no timeout". pbs.Config gains
IdleConnTimeout for tests only.

hub and proxmox carried the same missing default and are corrected here for
consistency. Neither contributed to the ep0 leak -- both are built once per
process and neither talks to ep0:8007.

Tests count connections SERVER-side and model the abandonment, so they pin
the consequence rather than the field. Two red-proofs, both seen failing:
removing the timeout -> "still holds 5 open connection(s), want 0";
DisableKeepAlives -> "3 sequential requests over 3 connection(s), want 1"
(the leak test PASSES under that one -- it is the worse-fix guard that
catches it).

Not released: hand-installed on demo-hp only so demo-felhom stays the
control. CHANGELOG heading stays UNRELEASED until the publish is authorised.

REUSE.md

@@ -86,6 +86,7 @@
 | `Client.PoolAddVMID` | internal/proxmox/mutate.go | `PoolAddVMID(ctx, pool, vmid) error` | re-assert pool membership after a restore-over-existing (campaign-2 R2) | SYNC (no UPID, don't WaitTask); PVE `PUT /pools` is additive (merge, not replace) — `delete=1` removes; idempotent (already-member swallowed); needs `Pool.Allocate` at `/pool/<pool>`. `pct restore --pool` sets membership only at CREATE — a restore over an existing vmid drops it, so bring-up re-asserts post-restore |
 | `TLSConfig.build` / `normalizeFingerprint` | internal/proxmox/tls.go | `build() (*tls.Config, error)` | PVE leaf-cert SHA-256 pinning | No insecure default |
 | `pinnedTLS` | internal/pbs/pin.go | `pinnedTLS(fingerprint) (*tls.Config, error)` | PBS leaf pinning | Same model as PVE; 64-hex fingerprint normalized |
+| `httpx.NewTransport` | internal/httpx/transport.go | `NewTransport(tlsCfg, idleConnTimeout) *http.Transport` | **EVERY** hand-rolled `http.Transport` in this repo — pbs, hub and proxmox all pin TLS, so none can use `http.DefaultTransport` | **R-344: never inline `&http.Transport{TLSClientConfig: ...}` again.** A composite literal takes `IdleConnTimeout` **zero, which means retain idle connections FOREVER** — `http.DefaultTransport` sets 90s and a literal does not inherit it. Combined with a client rebuilt per cycle and dropped (`pbsTargetsFromPVE`), that stranded **388 sockets on ep0 in 46 h**, held open on BOTH sides. `idleConnTimeout <= 0` means **use the default**, never "no timeout". Returns a **FRESH** transport every call — a shared one would pool connections across differently pinned endpoints. Pinned by `internal/pbs/client_leak_test.go` (server-side connection counting) + `internal/httpx/transport_test.go` |
 | `hub.Client.Report` | internal/hub/client.go | `Report(ctx, *HostReport) (*ControlEnvelope, error)` | the heartbeat | Typed `TransportError`/`HTTPError`, never contain the bearer token |
 | `hub.Loop` + `MultiObserver` | internal/hub/loop.go | `NewLoop(...)`; `MultiObserver(obs...)` | resilient report loop + envelope fan-out | Errors logged, loop continues; interval clamped 60–3600 s |
 | `provision.BackHalf.Provision` | internal/provision/backhalf.go | `Provision(ctx, Input) (Result, error)` | guest bootstrap back-half | mint→render→0600 write→chown 100000:100000→`pct set` ro bind→onboot; token NEVER logged/returned. Bootstrap `local_api.endpoint` = the caller's `cfg.LocalAPI.ListenAddr` (main.go) — moving the agent bind to the island moves the guest dial for free (R-50, no template) |