feat: profile pictures — Gravatar fallback + custom upload

New users get a Gravatar-backed avatar automatically (computed from email
at signup); users can upload a custom photo instead via Settings, or
revert to the Gravatar/initials fallback. avatarUrl stays the single
resolved value (custom photo, OAuth photo, or precomputed Gravatar URL)
so every existing avatar-rendering spot across the app needs zero changes.

- users.hasCustomAvatar tracks whether avatarUrl is a real upload vs a
  computed Gravatar fallback, so "remove photo" knows what to revert to.
- New /api/v1/upload/avatar-presign route (session-scoped, generic —
  the existing recipe-photo presign route required a recipeId).
- CSP img-src needed www.gravatar.com added, or every browser blocks
  the fallback avatar outright.
- Settings page now reads avatarUrl from a fresh DB query instead of
  Better Auth's session object — the session cookie cache (5 min TTL)
  was serving a stale image right after upload, showing the initials
  fallback until the cache happened to expire.

Verified locally: signup auto-sets a Gravatar URL, upload persists
across reload, remove correctly reverts to Gravatar/initials.

apps/web/app/(app)/settings/page.tsx

@@ -12,7 +12,7 @@ export default async function SettingsPage() {
 
   const dbUser = await db.query.users.findFirst({
     where: eq(users.id, session.user.id),
-    columns: { bio: true, privateBio: true, isPrivate: true },
+    columns: { bio: true, privateBio: true, isPrivate: true, hasCustomAvatar: true, avatarUrl: true },
   });
 
   return (
@@ -20,11 +20,12 @@ export default async function SettingsPage() {
       user={{
         name: session.user.name,
         email: session.user.email,
-        image: session.user.image ?? null,
+        image: dbUser?.avatarUrl ?? null,
         locale: (session.user as { locale?: string }).locale ?? "en",
         bio: dbUser?.bio ?? null,
         privateBio: dbUser?.privateBio ?? null,
         isPrivate: dbUser?.isPrivate ?? false,
+        hasCustomAvatar: dbUser?.hasCustomAvatar ?? false,
       }}
     />
   );