feat: file transcription core (ffmpeg convert, whisper segments, srt/vtt export)

[ElbertePlinio]

Refs #30 — PR 1 of 2 (core library slice; PR 2 adds the Tauri commands + UI).

What changed

• src/engine/media.rs (new): ffmpeg resolution with actionable error, cancellable conversion of any audio/video file to 16 kHz mono s16 WAV (partial output + logs removed on cancel/failure), WAV duration reader for history.
• src/engine/transcript.rs (new): FileSegment, whisper --output-json parsing (drops non-speech markers), TXT/SRT/VTT formatters — pure and unit-tested, so exports can be regenerated from History at any time.
• src/engine/stt.rs: transcribe_file_with_cancel — whisper-cli with --output-json --print-progress --no-prints, progress callback parsed from the stderr log tail (UTF-8-lossy), same cancel/cleanup discipline as the existing dictation path (untouched).
• src/history.rs: additive migration adding nullable source_file + segments_json (guarded ALTER TABLE for legacy DBs, columns in CREATE TABLE for fresh ones); insert/list updated.
• src-tauri/src/engine.rs: dictation insert passes None for the two new fields — no behavior change.

Tested

• cargo test --workspace --locked --all-targets: 74 passed (baseline 67; new tests cover JSON parsing, SRT/VTT exact output incl. >1 h rollover, WAV duration, progress-line parsing, legacy-DB migration + fresh-DB round-trips).
• Live checks against the installed whisper-cli: output filename convention (<prefix>.json appending), progress = N% stderr format, and that --no-prints keeps progress lines.

Not tested yet

• Real end-to-end media→history flow (needs the PR 2 command layer; issue Transcribe a file: drop audio/video, get transcript + SRT/VTT export #30's validation matrix — WAV/MP3/MP4, cancel orphan check, local-only — runs there).

No user-facing change; release notes drafted under Internal in docs/releases/UNRELEASED.md.

[ElbertePlinio]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🎉

Reviewed commit: 6c07135a2b

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".