feat(telemetry): readable Sentry errors, Python tracebacks, and diagnostic mode

Keeps a real, redacted error message instead of "Error: Error", surfaces Python tracebacks in Sentry as a vetted context, and adds an opt-in SNAPOTTER_SENTRY_DIAGNOSTIC verbose mode plus SNAPOTTER_SENTRY_DSN_OVERRIDE. The default fleet path ships nothing on the never-collect list; raw detail is reachable only via the opt-in flag. Also classifies Redis OOM/READONLY replies as operational and removes a ReDoS in stack-frame extraction.

tests/unit/api/capture-path.test.ts

@@ -39,6 +39,7 @@ vi.mock("@sentry/node", () => ({
 
 vi.mock("../../../apps/api/src/lib/analytics-gate.js", () => ({
   analyticsEnabled: () => true,
+  sentryDiagnostic: () => false,
 }));
 
 beforeEach(() => {