fix(sentry): sample traces and session replays to stay under quota

[giladresisi]

What kind of change does this PR introduce?

Config change (backend, orchestrator, frontend Sentry init). In initialize.sentry.ts the fixed tracesSampleRate: 1.0 becomes a tracesSampler: requests whose path contains /public/v1/analytics/ are sampled at 1%, all other backend and orchestrator traces (public API, MCP, app API, background jobs) at 20%, and an incoming sampled parent trace keeps its decision. Frontend tracesSampleRate goes from 1.0 to 0.2 (browser and Next.js server), and replaysSessionSampleRate from 1.0 to 0.4. Error events are not sampled, replaysOnErrorSampleRate stays 1.0, and self-hosted setups without a DSN are unaffected.

Why was this change needed?

Every trace and every browser session was sent to Sentry, and at the current rate we are projected to exceed our plan's span and replay quotas this billing period unless usage drops significantly.

Current usage (24h measured on 2026-09-15):

• Spans: about 77M/day, roughly 2.3B/month. GET /public/v1/analytics/post/:postId alone is about 33M/day (43%), GET /public/v1/posts about 13M/day, MCP about 9M/day, frontend about 1.5M/day.
• Replays: about 4K sessions/day, roughly 120K/month.

Expected usage after this change (extrapolated from the same day):

• Spans: about 9M/day, roughly 270M/month (analytics ~0.3M/day, rest of backend ~8.4M/day, frontend ~0.3M/day), about 88% less.
• Replays: about 1.6K sessions/day, roughly 48K/month plus error sessions.

Sentry recommends lowering the trace sample rate for high-load production backends (https://docs.sentry.io/platforms/javascript/guides/nestjs/configuration/sampling/) and keeping on-error replays at 100% (https://docs.sentry.io/platforms/javascript/session-replay/).

Other information:

Rates were agreed with the team. Possible follow-ups, not in this PR: restoring the Redis cache read for post analytics and not sending spans for cached responses (could allow higher rates), and dropping Prisma internal child spans via ignoreSpans.

QA

1. Run an Express app with @sentry/nestjs and this tracesSampler, collecting transactions in beforeSendTransaction, and send 3,000 requests each to /public/v1/analytics/post/:id, /public/v1/analytics/:integration, /public/v1/posts, /mcp/:id and /user/self
2. Expect about 1% of the analytics requests and about 20% of the others to produce transactions (observed: 35, 32, 575, 580 and 556 of 3,000)
3. Send 200 requests with a sentry-trace header marked sampled; all 200 should produce transactions
4. Hit a route that throws 50 times; all 50 error events should be sent regardless of trace sampling
5. Open the frontend with a DSN set and read the Sentry client options in the console: tracesSampleRate 0.2, replaysSessionSampleRate 0.4, replaysOnErrorSampleRate 1
6. In a session whose sessionStorage.sentryReplaySession has sampled: "buffer", capture an exception; the replay for that session should appear in Sentry with 1 error (verified with the previous 0.25 session rate; the on-error path is unchanged)
7. After deploy, check the spans/day for the cloud project drops to roughly the expected numbers above

Checklist:

• I have read the CONTRIBUTING guide.
• I have signed the Contributor License Agreement (CLA) (ICLA for individuals, CCLA for entities).
• I confirm I have not used AI to submit this PR or generate code for it.
• I checked that there were no similar issues or PRs already open for this.
• This PR fixes just ONE issue
• I have filled in the QA section above with real steps to verify this change.

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