Fix seeder: origin mismatch + robust Dockerfile

Two root causes for seeding not working on production:

1. Origin mismatch: The seeder writes content IDs as
   http://localhost:7777/content/... but the app was using
   window.location.origin (the user's actual browser URL) to
   query the relay. Introduced VITE_CONTENT_ORIGIN env var
   baked into the Docker build so both sides use the same origin.

2. Dockerfile.seed fragility: Replaced --omit=dev + global tsx
   with a cleaner approach that strips sharp from package.json
   (the only native dep that fails on Alpine) then does a full
   npm install, ensuring tsx/esbuild and all applesauce deps
   resolve correctly.

Also improved wait-for-relay to accept any HTTP response (some
relays return 4xx for plain GET) and increased max attempts.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/composables/useContentDiscovery.ts

@@ -170,11 +170,10 @@ export function useContentDiscovery() {
       if (!effectiveAlgo) return contents
 
       const statsMap = contentStatsMap.value
-      const origin = window.location.origin
 
       // Build entries array: [Content, stats] for each content item
       const withStats: [Content, ContentStats][] = contents.map((c) => {
-        const externalId = `${origin}/content/${c.id}`
+        const externalId = getExternalContentId(c.id)
         return [c, statsMap.get(externalId) || EMPTY_STATS]
       })