Web export missing per-bubble token counts and aggregated cost metadata

[wpak-ai]

Problem

The web bulk export (POST /api/export) produces Markdown files that are missing
several fields visible in the UI for each conversation:

Per-message metadata line — the UI shows token counts (input, output, cached)
for each AI bubble in the workspace view, but the exported Markdown omits them
entirely.

Frontmatter — the following aggregated fields shown in the UI tab summary are
absent from the exported frontmatter:

• workspace_name (human-readable, URL-decoded folder name)
• total_input_tokens
• total_output_tokens
• total_cached_tokens
• total_cost_usd

This violates the "what you see is what you get" principle: content clearly
visible in the browser disappears in the exported file.

Fix

• Extract tokenCount from each raw bubble in cursorDiskKV and include
  inputTokens, outputTokens, cachedTokens in the per-bubble data
  structure, then render them as _In: X / Out: Y / Cached: Z_ in the
  Markdown metadata line.
• Aggregate the above into total_input_tokens, total_output_tokens,
  total_cached_tokens for the frontmatter.
• Derive total_cost_usd from the composer's usageData.cost /
  usageData.estimatedCost field.
• Always include workspace_name (URL-decoded display name) in the frontmatter
  alongside the existing workspace slug.

Affected file

api/export_api.py