Stabilize NoteFlow prototype and add CI

[Harryphan72007]

Summary

• establish a conventional main baseline and add backend/frontend CI
• remove generated uploads, exports, evidence dumps, and unused UI scaffolding
• replace machine-specific paths with portable configuration
• consolidate documentation around architecture, status, privacy, and limitations
• label all interface fixtures as synthetic demo data
• make heavyweight OCR dependencies optional with a clear runtime error

Verification

• python -m pytest backend/tests (19 passed)
• pnpm lint
• pnpm typecheck
• pnpm test (2 passed)
• pnpm build

Migration note

The repository default branch currently remains ix/complete-real-integrations. After review and merge, switch the default branch to main; no release tag is created until that migration is complete.