fix: persist movie format flags, add VHS and Digital formats

[mforce]

Summary

Fixes movie format checkboxes not persisting after save, and adds two new formats: VHS and Digital.

Tracking issue: Closes #85

Bug fix

The frontend sends formats as a bitwise integer (e.g. 3 for DVD | Blu-ray) via fetch. The server's global JsonStringEnumConverter expected a string like "Dvd, BluRay" and silently fell back to None (0) when it received a number. Every format selection was lost on save.

Fix: Changed MovieDto.Formats from MovieFormat to int so the integer passes through JSON unchanged. Internal casts in ToDto/ApplyDto keep the domain model intact.

New formats

Added Vhs = 8 and Digital = 16 to the [Flags] MovieFormat enum. They appear as independent toggle buttons on the movie form alongside the existing DVD / Blu-ray / UHD Blu-ray options.

Verification

• Server: 366/366 tests pass (3 new)
• Client: build clean, 106/106 tests pass
• ci-local: OK

Files changed

File	Change
Collectify.Domain/Enums/MovieFormat.cs	Added Vhs, Digital
Collectify.Api/Endpoints/MoviesEndpoints.cs	DTO Formats → int, cast internally
client/services/types.ts	Added VHS and Digital to type + flags array
tests/Api/MoviesEndpointsTests.cs	3 new tests for integer round-trip and new formats

[mforce]

One doc follow-up before this disappears into the merge mist:

docs/data-model.md still says “All enums serialize as strings (already configured globally via JsonStringEnumConverter).” This PR intentionally makes movie formats cross the wire as an integer bitmask instead, and the issue is specifically about that contract. Please update the docs to call out MovieFormat/movie formats as the exception, otherwise the next pass through the data model docs could easily reintroduce the string enum behavior.

Code-wise this looks good to me: the DTO boundary now returns numeric flags, the new values are power-of-two flags, and the raw JSON integration tests cover create/update/new-format round-trips.