feat: harden support, localize knowledge and add operator availability [skip ci]

Source-only publication of reviewed commit 64e96a7f423673dc29ad56485be9fc79bce8af1c.

Passed in isolation: focused frontend/backend tests, race checks, TypeScript typecheck, lint, Go compilation, Flowgram build, and deterministic SDK build/tests.

Not accepted as a production release: Next.js export/native assembly remains blocked by environment memory limits; browser interaction and MySQL staging acceptance remain unverified. No production server, database/schema, runtime configuration, deployment, release tag, or workflow execution was performed.

Expected source tree: 0a4a7528e9.

web/lib/message-file-identity.ts

@@ -0,0 +1,13 @@
+/**
+ * Retry identity for a selected attachment. A file picker returns a fresh File
+ * object on reselection; metadata alone can also collide across different bytes.
+ */
+export async function getMessageFileIdentity(file: Pick<File, "name" | "type" | "arrayBuffer">) {
+  const bytes = await file.arrayBuffer()
+  const subtle = globalThis.crypto?.subtle
+  // Self-hosted HTTP deployments may lack WebCrypto. Exact bytes provide a
+  // collision-free in-memory fallback, rather than silently weakening the key.
+  const digest = subtle ? await subtle.digest("SHA-256", bytes) : bytes
+  const key = Array.from(new Uint8Array(digest), (byte) => byte.toString(16).padStart(2, "0")).join("")
+  return JSON.stringify([file.name, file.type, subtle ? "sha256" : "bytes", key])
+}