Add cache control to Files API static assets

Brand assets served at /api/files/v1/static had no cache headers.
Introduce brand.Assets to own the embedded filesystem, content-hash
ETags, and HTTP serving. Responses now carry Cache-Control and ETag
so clients can cache and revalidate; stable email URLs stay
revalidatable (max-age=3600, no immutable).

Replace hardcoded Default*Path constants with StaticPathPrefix,
logical filename constants, and StaticPath(). NewAssets validates
required assets at startup so a rename fails fast instead of 404ing
in sent emails. The files handler keeps routing and 404 rendering;
ServeAssets sets cache headers and serves the file.

Signed-off-by: Ludovic Vielle <ludovic@probo.com>

packages/emails/emails.go

@@ -66,10 +66,10 @@ func DefaultPresenterConfig(baseURL string) PresenterConfig {
 	return PresenterConfig{
 		APIBaseURL:                      baseURL, // always API base URL
 		BaseURL:                         baseURL, // can change to custom domain when needed
-		PoweredByLogoPath:               brand.DefaultPoweredByLogoPath,
+		PoweredByLogoPath:               brand.StaticPath(brand.PoweredByLogo),
 		SenderCompanyName:               "Probo",
 		SenderCompanyWebsiteURL:         "https://www.probo.com",
-		SenderCompanyLogoPath:           brand.DefaultSenderCompanyLogoPath,
+		SenderCompanyLogoPath:           brand.StaticPath(brand.SenderCompanyLogo),
 		SenderCompanyHeadquarterAddress: "Probo Inc, 490 Post St, STE 640, San Francisco, CA, 94102, US",
 	}
 }