feat(inventory): multi-colour gradients, transparency, visual effects (#1154)

Spool and color_catalog rows carry extra_colors (comma-separated hex
  stops) and effect_type (14 visual variants: surface effects, sheen,
  structural). The shared FilamentSwatch component renders gradient,
  conic, effect overlay, and alpha-checkerboard consistently across the
  inventory grid, table, group banner, card, ColorSection preview, and
  catalog editor. Catalog hex_color accepts #RRGGBBAA so catalog entries
  can carry transparency too.

  The paste field accepts the exact format 3dfilamentprofiles.com puts on
  its filament details pages, so users can copy a multi-colour combo
  directly. The effect dropdown spans the full filament-variant
  vocabulary -- surface effects (sparkle/wood/marble/glow/matte), sheen
  variants (silk/galaxy/rainbow/metal/translucent), and structural
  variants (gradient/dual-color/tri-color/multicolor). None of these
  fields touch MQTT/firmware -- pure visual hint.

  Spool group-key extended to include extra_colors + effect_type so
  "Group similar" no longer collapses visually distinct spools.

  Migrations: 4 idempotent ALTER TABLE ADD COLUMN (Postgres-safe), plus
  ALTER COLUMN hex_color TYPE VARCHAR(9) on Postgres only (SQLite ignores
  VARCHAR length).

  Tests: 42 new backend (35 unit + 7 integration), 20 new frontend (14
  FilamentSwatch + 3 ColorCatalogSettings + 3 InventoryPageGrouping
  regression). 3522 backend + 1582 frontend tests pass; ruff clean.
  Localised across all 8 UI locales.

frontend/src/__tests__/components/SpoolFormModal.test.tsx

@@ -60,6 +60,8 @@ const existingSpool: InventorySpool = {
   brand: 'Polymaker',
   color_name: 'Red',
   rgba: 'FF0000FF',
+  extra_colors: null,
+  effect_type: null,
   label_weight: 1000,
   core_weight: 250,
   core_weight_catalog_id: null,