Trend data can contain only one day due to insufficient collection

[simplerjiang]

• Source: .automation/buglist.md #57 runtime data watch.
• Impact: Trend-based analysis can look broken or under-informed.
• Repro:
  1. Query a symbol with sparse collection history.
  2. Inspect trend window length.
• Expected: UI/API explain insufficient collection or backfill enough data.
• Actual: Runtime watch records one-day trend data.
• Claim: Assign market-data owner.
• Repair: Add minimum-window checks and user-visible fallback.
• Verification: API tests for sparse trend data plus UI empty/limited-state check.

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