feat(learning): import local roadmap packs into the catalogue

[OthmaneZ05]

Summary of Changes

Roadmaps no longer have to live in the repo's roadmaps/ directory. This PR adds local import of roadmap packs:

• Catalogue merge: the learning catalogue now scans two directories on every request — the shipped roadmaps/ (source builtin) and ~/.torollo/roadmaps/ (source imported). Dropping a valid file in the user folder makes it appear on refresh, no restart.
• Import endpoint: POST /api/learning/roadmaps/import accepts a raw .json roadmap or a .zip archive of them (magic-byte detection, adm-zip, entry-count/size caps). Every file is Ajv-validated before anything is written and the response is a per-file report (imported / rejected with exact field-level errors / ignored), so a typo in a hand-written roadmap is an actionable message, not a silent failure.
• No shadowing: progression is keyed on roadmap id, so an import colliding with a shipped (id, language) is refused with a clear message — both at import time and at scan time for hand-dropped files. Installed files are named <id>.<language>.json, so re-importing a fixed pack updates in place (updated in the report).
• Frontend: an Import roadmaps button on the Learning page with a report modal, an "Imported (EN)" badge on catalogue cards, and imported roadmaps bypass the UI-language filter (an EN-only pack stays visible in a FR UI — you installed it on purpose). i18n EN/FR.
• Docs: new docs/local-roadmaps.md, plus updates to docs/learning-api.md, docs/roadmap-format.md and the README.

Roadmaps remain pure data — importing never executes anything from the file.

Types of Changes

• New feature / node type addition
• Bug fix (non-breaking change resolving an issue)
• Refactoring / structural cleanup
• Documentation update

Verification & Testing

Automated Checks

• Run npm run lint successfully with no errors
• Run npm run build successfully with no compilation errors
• Run npm test successfully (all tests pass)

Backend: 470 jest tests passing (incl. 12 new import-service tests with real zip archives, 6 controller tests, merged-catalogue service tests). Frontend: 310 vitest tests passing (incl. language-filter bypass and import-button tests). Both builds and lints clean.

Manual Verification

End-to-end against real Docker, on an isolated backend (HOME scratch dir, compiled dist/):

• Imported a .zip containing a valid roadmap, an invalid one and a README → 200 with 1 imported, 1 rejected with field-level errors, 1 ignored; catalogue then listed the 6 shipped roadmaps in curated order plus the imported entry marked imported.
• Played the imported roadmap end to end: fail-first validation on both steps, created the real ubuntu + postgres containers, both steps passed, replay stable, progression 2/2 persisted, completion reached.
• Collision with shipped resilient-three-tier → 422 with the explanatory refusal.
• Real UI (Playwright): import button + report modal, "Imported" badge, re-import shows updated, briefing page of the imported roadmap works, and the EN-only imported roadmap stays visible with the UI switched to French.
• Broken upload with a wrong Content-Type gets an explicit 400 telling the client to use application/octet-stream.

Checklist

• My code follows the repository's code style and lint standards
• I have updated the documentation or instructions if necessary
• All unit and integration tests are passing