fix(admin): accept null for nullable settings fields in PUT endpoint (#120)

The Fastify JSON Schema and Zod validation for PUT /api/admin/settings
only accepted string for communityDescription, communityLogoUrl,
primaryColor, and accentColor — but the GET response returns null for
these fields when unset. The frontend round-trips the null values back,
causing Fastify's AJV to reject the request with a 400 before the
handler runs. Align both validation layers with the database schema by
adding null as an accepted type, matching the existing pattern used by
jurisdictionCountry.

Fixes barazo-forum/barazo-workspace#TBD

src/routes/admin-settings.ts

···
                 items: { type: 'string', minLength: 1, maxLength: 30 },
                 minItems: 1,
               },
-              communityDescription: { type: 'string', maxLength: 500 },
-              communityLogoUrl: { type: 'string', format: 'uri' },
+              communityDescription: { type: ['string', 'null'], maxLength: 500 },
+              communityLogoUrl: { type: ['string', 'null'], format: 'uri' },
               primaryColor: {
-                type: 'string',
+                type: ['string', 'null'],
                 pattern: '^#(?:[0-9a-fA-F]{3,4}|[0-9a-fA-F]{6}|[0-9a-fA-F]{8})$',
               },
               accentColor: {
-                type: 'string',
+                type: ['string', 'null'],
                 pattern: '^#(?:[0-9a-fA-F]{3,4}|[0-9a-fA-F]{6}|[0-9a-fA-F]{8})$',
               },
               jurisdictionCountry: { type: ['string', 'null'] },

src/validation/admin-settings.ts

···
     .string()
     .trim()
     .max(500, 'Community description must be at most 500 characters')
+    .nullable()
     .optional(),
-  communityLogoUrl: z.url('Community logo must be a valid URL').optional(),
+  communityLogoUrl: z.url('Community logo must be a valid URL').nullable().optional(),
   primaryColor: z
     .string()
     .regex(hexColorPattern, 'Primary color must be a valid hex color (e.g., #ff0000)')
+    .nullable()
     .optional(),
   accentColor: z
     .string()
     .regex(hexColorPattern, 'Accent color must be a valid hex color (e.g., #00ff00)')
+    .nullable()
     .optional(),
   jurisdictionCountry: z
     .string()