fix(inventory): malformed rgba no longer bricks the Filaments page (#1055)

A single legacy spool with a 7-char rgba ('FFFFFFF', missing one F)
  caused GET /api/v1/inventory/spools to 500 with a pydantic
  ResponseValidationError, leaving the reporter with a blank Filaments
  page and "Add Spool" silently failing. Root cause spans three layers:

  1. Write path: SpoolUpdate.rgba had no pattern constraint (only
     SpoolCreate did), so PATCH could plant malformed values in the DB.
  2. Frontend: ColorSection hex input's `val.length <= 6 ? 'FF' : ''`
     emitted 7-char rgba for 5-char input (XXXXX + FF = 7) and for
     7-char typed input (no alpha appended).
  3. Read path: SpoolResponse inherited the write-side pattern, so a
     single bad row 500'd the entire list endpoint instead of being
     tolerated through serialize.

  SpoolUpdate.rgba now carries the same ^[0-9A-Fa-f]{8}$ pattern as
  SpoolCreate. The hex input emits a fully-formed 8-char RRGGBBAA on
  every keystroke — 8-char paste passes through, 7-char drops the
  stray, shorter input pads RGB with '0' and appends FF alpha.
  SpoolResponse.rgba is now Optional[str] with no pattern — write-side
  validation is the right place for format rules; responses must
  tolerate historical rows.

  Tests: 16 schema tests (SpoolCreate/Update reject, SpoolResponse
  tolerate), 7 frontend tests covering every input length 0–8 plus
  non-hex strip. A user who already has a bad row in their DB now sees
  it render with a default color instead of having to hand-edit SQLite.

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

@@ -348,6 +348,73 @@ describe('SpoolFormModal weightTouched', () => {
     expect(payload).toHaveProperty('cost_per_kg', null);
   });
 
+  it('normalizes a malformed legacy rgba on edit-form load so PATCH is not rejected (#1055)', async () => {
+    // #1055 regression guard: a spool with a legacy 7-char rgba (e.g. 'FFFFFFF')
+    // was editable in the UI but any save 422'd because SpoolUpdate now enforces
+    // the 8-char pattern. The form must sanitize the loaded value to a valid
+    // default so users can edit unrelated fields without being forced to fix
+    // a color they may not even have noticed was broken.
+    const spoolWithBadRgba: InventorySpool = {
+      ...existingSpool,
+      rgba: 'FFFFFFF', // 7 chars — the exact #1055 trigger pattern
+    };
+
+    render(
+      <SpoolFormModal
+        isOpen={true}
+        onClose={vi.fn()}
+        spool={spoolWithBadRgba}
+        currencySymbol="$"
+      />
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText('Edit Spool')).toBeInTheDocument();
+    });
+
+    const saveButton = screen.getByRole('button', { name: /save/i });
+    fireEvent.click(saveButton);
+
+    await waitFor(() => {
+      expect(api.updateSpool).toHaveBeenCalledTimes(1);
+    });
+
+    const [, payload] = vi.mocked(api.updateSpool).mock.calls[0];
+    // The PATCH payload must carry a valid 8-char rgba — never the raw 7-char
+    // value loaded from the stale DB row.
+    expect(payload).toHaveProperty('rgba');
+    expect(typeof (payload as { rgba: unknown }).rgba).toBe('string');
+    expect((payload as { rgba: string }).rgba).toMatch(/^[0-9A-Fa-f]{8}$/);
+  });
+
+  it('preserves a valid existing rgba on edit (no forced default)', async () => {
+    // Sanity: the normalization only kicks in for malformed values. A valid
+    // 8-char rgba must round-trip untouched so untouched edits don't quietly
+    // reset a user's chosen color.
+    render(
+      <SpoolFormModal
+        isOpen={true}
+        onClose={vi.fn()}
+        spool={existingSpool} // rgba = 'FF0000FF' (valid)
+        currencySymbol="$"
+      />
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText('Edit Spool')).toBeInTheDocument();
+    });
+
+    const saveButton = screen.getByRole('button', { name: /save/i });
+    fireEvent.click(saveButton);
+
+    await waitFor(() => {
+      expect(api.updateSpool).toHaveBeenCalledTimes(1);
+    });
+
+    const [, payload] = vi.mocked(api.updateSpool).mock.calls[0];
+    expect((payload as { rgba: string }).rgba).toBe('FF0000FF');
+  });
+
   it('displays correct catalog name when duplicates exist', async () => {
     const spoolWithCatalogId: InventorySpool = {
       ...existingSpool,