Add Obsidian export, cache-busting, CDN pinning, and review fixes

[dmitry-kostin]

Summary

• Obsidian export — one-click export to Obsidian vault via obsidian://new URI scheme with vault config UI, clipboard fallback, and localStorage persistence
• Static asset cache-busting — main.py rewrites /static/app.js and /static/style.css URLs with ?v={mtime} stamps
• CDN pinning — marked library loaded with SRI integrity hash
• Summary title as markdown heading — prefix changed from "Title\n\n" to "# Title\n\n" in both summarize endpoints
• save_summary error handling — both summarize and demo_summarize return 500 on save failure
• Test improvements — remove redundant tmp_path/monkeypatch setup, use AsyncMock for summarize_text, add test_delete_cascades_to_summary, verify mock call args

Stats

8 files changed, ~469 lines added

Test plan

• poetry run pytest — all unit + endpoint tests pass
• Manual: export a transcript to Obsidian, verify vault config prompt and URI open
• Manual: verify static assets reload after code changes (cache-busting)
• Manual: summarize a transcript, verify # Title heading in summary

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