Add Trip Editor rich-notes persistence contracts

[stef-k]

Summary

Implements #297 Batch 5 only: Trip Editor request-boundary rich-notes normalization and persistence coverage.

• Adds EditorRichNotesRequestHtml as a Trip Editor request-boundary helper, not a broad app sanitizer.
• Wires metadata, region, place, area, and segment editor request parsers through the helper before persistence.
• Backend tests cover canonical HTML behavior for formatting, lists, links, alignment, images, proxy URL unwrapping, data-image handling, unsafe attribute stripping, and trailing helper paragraph removal.
• Adds controller-level metadata persistence proof through parser/service/database.
• Adds one real Playwright metadata rich-notes persistence spec that saves through the actual PATCH endpoint and verifies API reread/reload behavior.

Claim Boundary

Existing rich-notes Playwright specs remain mocked/fixture-backed and validate editor UX, client canonicalization, request shape, image-dialog behavior, and draft behavior only. They are not backend persistence proof.

The real persistence proof in this PR is limited to the new metadata rich-notes persistence spec plus backend request-boundary tests for the shared parser path.

Out Of Scope

• Batch 6 Development/published production asset smoke.
• Legacy/public/mobile HTML sanitization redesign.
• Broad app-wide sanitizer replacement.

Validation

• Focused helper tests passed.
• Focused helper + metadata persistence tests passed.
• Focused rich-notes Playwright specs passed.
• Playwright list passed with 117 tests listed.
• dotnet build passed.
• dotnet test passed, 1547 tests.
• LOC check passed under hard cap; warnings only.
• git diff --check origin/main...HEAD passed.
• tracked artifact scan: clean.