Prepare build-time static asset syncing without touching page templates

Constraint: Static images need to come from remote sources before log sync and build
Rejected: Per-file source mapping | directory-grouped manifests are easier to maintain
Confidence: high
Scope-risk: narrow
Directive: Prefer grouped target_dir manifests and keep page-facing paths stable
Tested: npm run assets:sync; npm run build; bash -n scripts/deploy-homepage.sh; npx tsc --noEmit --pretty false --project tsconfig.json
Not-tested: Real Seafile download against production URLs

.env.example

@@ -34,6 +34,7 @@ SEAFILE_TOKEN=
 # Relative paths are resolved from the repo root.
 # -----------------------------------------------------------------------------
 SYNC_OUTPUT_DIR=src/data/generated
+STATIC_ASSET_MANIFEST=src/content/static-assets/index.json
 
 # -----------------------------------------------------------------------------
 # Optional behavior
@@ -41,6 +42,7 @@ SYNC_OUTPUT_DIR=src/data/generated
 # falling back to cached/seed data.
 # -----------------------------------------------------------------------------
 STRICT_SYNC=false
+STATIC_ASSET_SYNC_STRICT=false
 
 # -----------------------------------------------------------------------------
 # Optional future behavior
@@ -57,3 +59,4 @@ GITEA_REQUEST_TIMEOUT_MS=15000
 GITEA_REQUEST_CONCURRENCY=5
 
 SEAFILE_REQUEST_TIMEOUT_MS=15000
+STATIC_ASSET_REQUEST_TIMEOUT_MS=15000