Add indexed, timestamped, cursor-paginated instance listing APIs

[Copilot]

df.instances listing was expensive for external clients because chronological reads had no supporting index, df.list_instances had no server-side pagination, and timestamps were omitted from the response. This made efficient page-by-page UI listing and metadata display harder than necessary.

• Schema/index changes

  • Added a chronological index for stable keyset scans:
    • idx_instances_created_at_desc_id on (created_at DESC, id).

• df.list_instances contract update

  • Extended return shape to include:
    • created_at
    • completed_at
  • Switched ordering to ORDER BY created_at DESC, id DESC for deterministic chronological output.
  • Replaced silent limit_count truncation with explicit bound validation.

• New paginated API

  • Added df.list_instances_paginated(status_filter, limit_count, after_cursor).
  • Uses keyset pagination on (created_at, id) with limit + 1 lookahead.
  • Returns:
    • instance fields + timestamps
    • total_count
    • next_cursor (or NULL when exhausted)
  • Supports optional status_filter under existing RLS behavior.

• Privilege/documentation updates

  • Added df.list_instances_paginated(text, integer, text) to df.grant_usage(...) function grants.
  • Updated user guide and changelog for new/expanded monitoring surfaces.

-- First page
SELECT * FROM df.list_instances_paginated(NULL, 20, NULL);

-- Next page
SELECT * FROM df.list_instances_paginated(
  NULL,
  20,
  '2026-01-01 12:00:00+00:00|a1b2c3d4'
);