fix(setup): tolerate reserved memory in the RAM preflight, add OMNIVOICE_RAM_PREFLIGHT=0 escape hatch (#1621)

* fix(setup): tolerate reserved memory in the RAM preflight, add OMNIVOICE_RAM_PREFLIGHT=0 escape hatch (#1618)

An "8 GB" machine reports ~7.8 GB usable (firmware/iGPU/kernel
reservations), so comparing OS-reported RAM against the marketing-size
8 GB threshold hard-blocked exactly the boundary hardware the minimum is
meant to admit — with no way past the wizard. Both thresholds are now
compared with a 7% reserved-memory allowance, and
OMNIVOICE_RAM_PREFLIGHT=0 downgrades a genuine fail to a warning for
users who accept the OOM risk (same opt-out shape as
OMNIVOICE_ASR_VRAM_PREFLIGHT).

Regression tests: backend/tests/test_ram_preflight_1618.py.
Docs: troubleshooting §1c.

* review: hermetic preflight stubs in tests; correct the escape-hatch doc

Greptile P1: the Settings panel can't set OMNIVOICE_RAM_PREFLIGHT (and the
blocker appears before setup completes anyway) — the doc now points at
PowerShell / shell env only.
CodeRabbit: stub _network_check and media_tools.summary so each RAM
assertion stays fast and offline (26s -> 6s locally).

docs/install/troubleshooting.md

@@ -138,6 +138,28 @@ failing outright.
 
 **Linked issue:** [#1185](https://github.com/debpalash/VoiceStudio/issues/1185)
 
+## 1c. Setup blocked: "System RAM … The app will OOM on first dub"
+
+**Symptom:** the setup wizard's System Check shows **System RAM** in red and
+"Resolve blockers to continue" stays disabled — often on an 8 GB machine that
+reports ~7.8 GB usable (firmware and integrated graphics reserve a slice of
+installed RAM).
+
+**Cause:** the preflight compares OS-reported RAM against the 8 GB minimum.
+Since [#1618](https://github.com/debpalash/VoiceStudio/issues/1618) the check
+tolerates that reserved-memory gap, so 8 GB-installed machines pass.
+
+**Fix:** update to the latest release. If your machine is genuinely below the
+minimum and you accept the out-of-memory risk (long dubs may crash), set
+`OMNIVOICE_RAM_PREFLIGHT=0` before launching — the hard block becomes a
+warning. On Windows run PowerShell
+`[Environment]::SetEnvironmentVariable('OMNIVOICE_RAM_PREFLIGHT','0','User')`
+and relaunch; on macOS/Linux export it in the shell that starts the app. (The
+in-app Settings panel can't help here — this blocker appears before setup
+completes.)
+
+**Linked issues:** [#1618](https://github.com/debpalash/VoiceStudio/issues/1618)
+
 ## 2. HF 401 / pyannote license not accepted
 
 **Symptom:** dubbing fails with `HfHubHTTPError: 401 Client Error: Unauthorized