Support an optional created_at on pipeline creation

[QnJ1c2kNCg]

PipelinePost takes an optional created_at in microseconds since the Unix epoch, applied to both the pipeline and its initial job. When omitted the inserts fall back to CURRENT_TIMESTAMP via COALESCE, preserving the existing database-clock behavior.

Testing
Tested a few different pipelines and validated via the UI:

gitlab/arroyo-all/arroyo  master [$!?]
❯ curl -sS -X POST http://localhost:5115/api/v1/pipelines \
  -H 'Content-Type: application/json' \
  -d '{
    "name": "backdated_test",
    "parallelism": 1,
    "created_at": 1583393889123456,
    "query": "create table impulse with (connector = '"'"'impulse'"'"', event_rate = '"'"'10'"'"');\nselect counter from impulse;"
  }' | jq '{id, name, created_at}'
{
  "id": "pl_RaUQ01xlBW",
  "name": "backdated_test",
  "created_at": 1583393889123456
}

gitlab/arroyo-all/arroyo  master [$!?]
❯ CREATED_AT=$(( $(date -v-7d +%s) * 1000000 ))
curl -sS -X POST http://localhost:5115/api/v1/pipelines \
  -H 'Content-Type: application/json' \
  -d "{
    \"name\": \"backdated_7d\",
    \"parallelism\": 1,
    \"created_at\": $CREATED_AT,
    \"query\": \"create table impulse with (connector = 'impulse', event_rate = '10');\\nselect counter from impulse;\"
  }" | jq '{id, created_at}'
{
  "id": "pl_OjeqgbnYlb",
  "created_at": 1784746618000000
}

gitlab/arroyo-all/arroyo  master [$!?]
❯ curl -sS -X POST http://localhost:5115/api/v1/pipelines \
  -H 'Content-Type: application/json' \
  -d '{
    "name": "wallclock_test",
    "parallelism": 1,
    "query": "create table impulse with (connector = '"'"'impulse'"'"', event_rate = '"'"'10'"'"');\nselect counter from impulse;"
  }' | jq '{id, created_at}'
{
  "id": "pl_pH9pETsKhs",
  "created_at": 1785351432000000
}

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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