Implement keep: self-hosted E2E encrypted secrets sync

Server: Express + better-sqlite3 (WAL), multi-recipient key-wrapping
per IMPLEMENTATION.md's design — vaults/recipients/vault_grants/
access_log. Two auth paths: ADMIN_PASSWORD header for recipient
management and revoke (pure metadata operations), signed-request
auth (Ed25519 signature over method+path+timestamp+body-hash) for
push/pull/grant, mirroring the spirit of this project family's other
signed-handshake patterns without naming them.

CLI: identity init/show/set-id, push/pull/grant/log, admin recipient
add/list/remove and revoke. Grant is a client-side crypto operation
(the granter unwraps the vault's current key locally and reseals it
for the new recipient) rather than a server-side operation, since the
server never holds an unwrapped key to grant with.

Verified end-to-end with two independent local identities against a
live server and separately against the built Docker image: register,
push, pull (granted and ungranted), grant without re-pushing, admin
revoke, a subsequent rotation confirming the revoked recipient stays
excluded, and rejection of missing/malformed signed-request auth.

Two real bugs caught during verification, not just written up:
- libsodium-wrappers' published ESM build does a relative import only
  resolvable under bundler-style resolution — broken under plain Node
  ESM. Fixed via createRequire to force the CJS build.
- Express's req.path inside a sub-router is relative to the mount
  point, which would have silently mismatched a client signing the
  full request path. Fixed by verifying against req.originalUrl.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,25 @@
+FROM node:22-alpine AS build
+WORKDIR /app
+# better-sqlite3 has a native addon with no prebuilt binary for this
+# platform/Node combo yet — build it from source.
+RUN apk add --no-cache python3 make g++
+COPY package*.json ./
+RUN npm install
+COPY . .
+RUN npm run build
+
+FROM node:22-alpine
+WORKDIR /app
+# Same native-addon rebuild needed for the production-only install below;
+# the toolchain is purged again afterward to keep the runtime image lean.
+RUN apk add --no-cache python3 make g++
+COPY package*.json ./
+RUN npm install --omit=dev && apk del python3 make g++
+COPY --from=build /app/dist ./dist
+
+# DATA_DIR (SQLite DB) is a mutable runtime volume — not baked into the
+# image, so redeploys don't clobber recipients/grants/vaults.
+ENV DATA_DIR=/app/data
+
+EXPOSE 3050
+CMD ["node", "dist/server/index.js"]