Fix MissingGreenlet 500 when creating a filament with a color

[wolfrage76]

POST /filament returns HTTP 500 when creating a filament that has a color.

Cause

create_filament() flushes a new Filament, then _apply_filament_colors() runs filament.filament_colors.append(...). On a freshly-flushed object whose filament_colors relationship was never loaded, the append triggers a lazy load, which raises sqlalchemy.exc.MissingGreenlet under the async (aiosqlite) engine:

greenlet_spawn has not been called; can't call await_only() here.

update_filament() is unaffected because _get_filament() eager-loads filament_colors via selectinload before _apply_filament_colors() runs.

Impact

Any client that creates a filament with a color hits this. It is reliably reproduced via Bambuddy: saving a profile on two spools that share a single filament makes Bambuddy create a per-spool filament (POST /filament), which 500s and surfaces in the UI as "Spoolman is not reachable".

Fix

Initialize the collection on the new object (filament.filament_colors = []) before add/flush, so the subsequent append() operates on a loaded, empty collection and performs no lazy IO. Mirrors how the update path already behaves.

Verified: POST /spoolman/api/v1/filament returns 200 and the filament is created with its color.