Return exact stable MCP timeline totals

[sarthakagrawal927]

What changed

• count PUBLIC timelines with the same optional search filter as the page query
• return one exact total on every page
• use a deterministic updated-time plus ID ordering
• cover first, middle, and terminal pages with a regression test

Root cause

The route exposed a lower-bound estimate (offset + items.length + 1) as total, so the value grew as clients paginated.

Impact

MCP clients can calculate terminal offsets and follow continuation metadata without contradictory totals.

Validation

• pnpm test — 483 passed
• pnpm typecheck
• pnpm lint (push hook)

Closes #91