OTLP nanosecond timestamp overflow in webapp event repository

[nicktrn]

Several places in the webapp multiply epoch milliseconds by 1,000,000 before converting to BigInt, which causes IEEE 754 precision loss (~256ns errors in ~0.2% of cases). The result exceeds Number.MAX_SAFE_INTEGER (~9e15) since epoch-ms * 1e6 is ~1.7e18.

The correct pattern is BigInt(ms) * BigInt(1_000_000) (multiplication after BigInt conversion). This already exists in convertDateToNanoseconds() in the same file - the buggy functions just didn't use it.

Affected locations:

1. apps/webapp/app/v3/eventRepository/common.server.ts:24 - getNowInNanoseconds()

   // Bug: multiplication in float-land
   return BigInt(new Date().getTime() * 1_000_000);
   // Fix:
   return BigInt(new Date().getTime()) * BigInt(1_000_000);

2. apps/webapp/app/v3/eventRepository/common.server.ts:38 - calculateDurationFromStart()

   // Bug:
   const duration = Number(BigInt($endtime.getTime() * 1_000_000) - startTime);
   // Fix:
   const duration = Number(BigInt($endtime.getTime()) * BigInt(1_000_000) - startTime);

3. apps/webapp/app/v3/eventRepository/index.server.ts:217 - recordRunDebugLog()

4. apps/webapp/app/v3/runEngineHandlers.server.ts:431 - retry event recording

Impact: Low. The errors are +-256 nanoseconds, which is unlikely to cause visible issues. But it's an easy fix and prevents any edge cases with span ordering.

Context: Found during compute workload manager PR review (#3114). The supervisor's msToNano() was fixed in that PR using a split approach that also preserves sub-ms precision from performance.now() arithmetic.

Linear: TRI-8269