feat(filaments): add spool-count column with sort/filter support

[akira69]

Summary

This PR hardens filament spool_count support so the filaments list can use it as a reliable inventory signal instead of a one-off display field.

Why

The useful part of spool_count is not just rendering a number. It needs to stay coherent across list queries, numeric filter semantics, spool relationship changes, and adjacent PRs that touch the same query plumbing.

Hardening In This PR

Backend

• Filament list queries treat spool_count as a first-class sortable and filterable field.
• Filament payload hydration now keeps spool_count populated on list, get, and update paths instead of leaving event payloads to drift to null or an implied 0.
• Distinct spool-count lookup stays numeric, including 0, without needing empty-string semantics.
• Spool relationship changes that affect counts are wired to refresh the related filament event path.
• The spool.filament_id index migration remains in scope to keep count lookups responsive.

Frontend

• The filament list exposes spool_count as a sortable, filterable numeric column.
• The filter path treats 0 as the real empty-state count rather than surfacing the generic "" option.
• The column display still normalizes missing values to 0 for safety, but the backend hydration path is now intended to keep that fallback from masking stale data.

Screenshots

Filament list with new Spool Count column visible. UI enabled to Hide/Show Spool Count column.

Sort by Spool Count (asc/desc).

Filter dropdown for Spool Count values (without "<empty>")

Testing Status

Targeted integration coverage was added for:

• filament get returning spool_count
• filament find filtering by spool_count=0
• filament find sorting by spool_count
• distinct values from GET /api/v1/filament/spool-count

Testing TODO

• Run the new filament integration slice covering spool_count get/find/filter/sort/distinct-value cases.
• Run the frontend build.
• Manually smoke-test the filament list column, numeric filter values, and clear-filters path.

Scope Boundary

This PR stays limited to spool-count correctness, query behavior, and integration hardening for the filament list path.
Manufacturer-level count columns and other list-query UX work remain out of scope.

Issues

• Fixes Filaments list improvements #201
• Fixes Inventory Management #207
• Fixes Number of spools (and filaments) shown directly in the overview/table (as a column with a number) #539
• Fixes Under Filaments should show QTY. #688
• Fixes Filament Count #742

Merge Notes

This branch still shares nearby query-plumbing touchpoints with adjacent filament-list PRs such as #858 and #862, so rebase/conflict cleanup may still be needed depending on merge order.