feat: add structured archive export

[chboishabba]

Summary

This PR adds a structured archive layer for Perplexity exports before Markdown/vector indexing, and improves long-thread extraction beyond the initial captured API page.

The exporter now writes itir.perplexity.thread.v1 JSON with normalized messages, stable source thread/message IDs, thread metadata, and captured API provenance. Markdown export remains available as an optional sidecar, and the existing vector/RAG flow can still be enabled when Markdown sidecars are present.

Why

The current Markdown-first flow is useful for reading and local vector search, but it makes dedupe, reimport, pagination validation, and downstream archive integrations harder than they need to be. A canonical thread/message export gives SQLite/MyChatArchive/ITIR-style tools a stable source of truth, while Markdown and vector indexes can be regenerated from that canonical layer.

A major practical issue is long Perplexity threads. The first browser-captured /rest/thread/<id> response can contain only the first page of entries. Without structured IDs and pagination checks, it is hard to know whether a run captured the whole thread, only page one, or duplicated page one repeatedly.

Pagination / long-thread behavior

This PR changes thread extraction so it does not stop at the first captured thread API response when Perplexity reports more pages:

• captures the exact /rest/thread/<thread-id> response for the conversation being exported
• follows next_cursor from page context with authenticated browser cookies
• appends additional entries when Perplexity returns genuinely new pages
• tracks entry identities across pages
• stops pagination if Perplexity replays only duplicate entries, instead of inflating the archive with repeated page-one content
• keeps the raw API response/entries in the structured export so downstream tools can audit what was captured

That last point is important: this does not pretend to bypass Perplexity/Cloudflare or guarantee all private webapp pagination works forever. It makes successful pagination useful, and failed/replayed pagination detectable and safe.

For cases where Perplexity's private API still only yields the first page but the UI download contains more content, this PR also adds a downloaded Markdown recovery path. npm run bundle:perplexity-downloads converts downloaded Markdown chunks into the same structured JSON shape so the recovered turns can attach to the same canonical thread downstream.

Changes

• Adds structured JSON export by default via EXPORT_STRUCTURED_JSON=true.
• Makes Markdown export optional via EXPORT_MARKDOWN=true.
• Adds normalized user/assistant message extraction from Perplexity API entries.
• Adds authenticated cursor pagination for thread detail responses.
• Adds duplicate-page detection so replayed first pages do not create fake extra messages.
• Adds bundle:perplexity-downloads for converting downloaded Markdown chunks into the same structured JSON shape.
• Defaults headful browser mode because Cloudflare/Turnstile makes headless unreliable for this flow.
• Adds unit coverage for config, structured file writes, pagination, duplicate-page replay, filename truncation, and downloaded Markdown bundling.

Validation

• npm run type-check
• npm run test:unit (25 tests passed)