feat: file blocks

Moderators block a file network-wide by its sha256, from a product's files or the console's new Blocks tab, which
first shows the products using it. Blocking removes those products (their creators are told and may appeal), puts
a `block` change in the changes feed of every catalog hosting it, and refuses uploads with it; moderators see who
tried. Unblocking lets it in again; the products it removed stay removed until restored.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/design.md

@@ -478,7 +478,8 @@ Phases for products, catalogs and imports (products.md, console.md, imports.md):
    (to look into with the importer spike).
 3. Cases (reports, appeals, reviews), file blocks and the catalogs' changes feed; notifications; API tokens;
    two-factor sign-in for sensitive permissions. Cases and notifications **done** (2026-11-05), without email, canned
-   reasons, moderator notes or second approval; file blocks, API tokens and two-factor sign-in next.
+   reasons, moderator notes or second approval. File blocks **done** (2026-11-05); API tokens and two-factor sign-in
+   next.
 4. Imports: grants, recorded grants, requests, unclaimed creators, the job queue, `t5de-importer` (after its spike).
 5. Charts and stats; mirroring between catalogs.