Add Filament label printing capability

[akira69]

As coded in PR #26

• Gives the ability for a user to print Filament-level labels, with QR codes linking to the filament page directly
• In my use case, I have swatches printed that are spool agnostic - "spool label" isn't enough.
• I personally use these swatches (note the labels and label content, which is inspiration for later features):

- and these holders on my containers/AMS

[akira69]

Status update: This is implemented in my fork at akira69#1 and is ready for upstream review. It adds a dedicated [/spools/:id/label] → [/filaments/:id/label] route giving filament-level print capability (QR codes linking to the filament page), parallel to the existing spool-label interface. The swatch use case shown above is exactly what it produces. Happy to discuss if this is something worth merging upstream.

[akira69]

now is PR #91 here