feat: add project bulk download and bulk video delete

Add a "Download project" / "Download selected" flow that builds a
server-side manifest of downloadable media, plus a selection mode with
bulk delete for project videos.

Gate viewer downloads behind a new project allowDownloads setting
(default off, opt-in). Admins can always download; enabling on a public
project allows anonymous visitors to download. Enforce the setting on
every download surface (manifest, version, asset, watch, video routes)
via canDownloadProjectMedia.

Add rate limits for the manifest endpoint, host allowlisting for direct
download URLs, and configurable file/byte caps.

Closes #16
Closes #19

app/(dashboard)/projects/[projectId]/settings/project-settings-page-client.tsx

@@ -85,6 +85,7 @@ export default function ProjectSettingsPageClient({ projectId }: ProjectSettings
     name: '',
     description: '',
     visibility: 'PRIVATE' as Visibility,
+    allowDownloads: false,
   });
 
   // Tag management state
@@ -109,6 +110,7 @@ export default function ProjectSettingsPageClient({ projectId }: ProjectSettings
             name: project.name || '',
             description: project.description || '',
             visibility: project.visibility || 'PRIVATE',
+            allowDownloads: project.allowDownloads ?? false,
           });
         }
       })
@@ -351,6 +353,48 @@ export default function ProjectSettingsPageClient({ projectId }: ProjectSettings
                   </div>
                 </div>
 
+                <div className="space-y-3 rounded-xl border p-4">
+                  <div>
+                    <Label className="text-sm font-medium">Project downloads</Label>
+                    <p className="text-sm text-muted-foreground mt-1">
+                      Allow viewers to download project files. Project admins can always download.
+                      When enabled on a public project, anyone with the link can download files
+                      without signing in.
+                    </p>
+                  </div>
+                  <button
+                    type="button"
+                    onClick={() =>
+                      setFormData((prev) => ({ ...prev, allowDownloads: !prev.allowDownloads }))
+                    }
+                    disabled={isSaving}
+                    className={`w-full flex items-center justify-between gap-4 p-4 rounded-xl border-2 text-left transition-all ${
+                      formData.allowDownloads
+                        ? 'border-primary bg-primary/5 ring-1 ring-primary/20'
+                        : 'border-border hover:border-border/80 hover:bg-accent/50'
+                    }`}
+                  >
+                    <div>
+                      <div className="font-medium">Allow viewer downloads</div>
+                      <div className="text-sm text-muted-foreground">
+                        Public and invited viewers can download files when enabled. On public
+                        projects this includes unauthenticated visitors.
+                      </div>
+                    </div>
+                    <div
+                      className={`shrink-0 w-5 h-5 rounded-full border-2 flex items-center justify-center ${
+                        formData.allowDownloads
+                          ? 'border-primary bg-primary'
+                          : 'border-muted-foreground/30'
+                      }`}
+                    >
+                      {formData.allowDownloads && (
+                        <div className="w-2 h-2 rounded-full bg-primary-foreground" />
+                      )}
+                    </div>
+                  </button>
+                </div>
+
                 {error && (
                   <div className="p-4 rounded-lg bg-destructive/10 border border-destructive/20 text-destructive text-sm">
                     {error}