fix(sandbox): address PR #250 review — SSRF guard, eviction safety, cleanup (#243)

Security:
- stash_page: reject path-traversal / percent-encoded srcs before the authed
  loopback fetch (resolveInternalFilePath), closing an SSRF/exfiltration hole
  where a crafted node.attrs.src could read an arbitrary internal GET endpoint
  into the anonymous sandbox.

Stability:
- stash_page: revert + recount mirrors FIFO-evicted by a later put in the same
  stash (no dangling sandbox refs, honest images.mirrored/failed); free image
  blobs if the final document put throws.
- Reject/clamp non-positive SANDBOX_TTL_MS to the 1h default (warn once).
- Log mirror failures unconditionally (console.warn, no blob bodies).

Cleanup / architecture:
- Remove dead expiresAt from SandboxPutResult.
- Centralize the /api/sb route in SANDBOX_ROUTE_SEGMENT/SANDBOX_API_PATH and
  move URL composition into SandboxStore.putAndLink; drop the duplicated sink
  closures and the now-unused EnvironmentService injection from McpService and
  AiChatToolsService.
- Un-export isInternalFileUrl; document the process-local (instance-bound)
  sandbox limitation in the tool description and .env.example.

Docs/tests:
- README/README.ru: 38 -> 39 tools + stash_page entry.
- Add traversal/normalize/recursion unit tests, stash self-eviction +
  doc-put-throw + empty/octet-stream mock tests, controller If-None-Match
  (wildcard/weak/list) + Cache-Control tests, and SANDBOX_TTL_MS validation
  tests. Regenerate packages/mcp/build.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

@@ -134,6 +134,10 @@ MCP_DOCMOST_PASSWORD=
 # SANDBOX_PUBLIC_URL is the base used to build those URLs; it MUST be reachable
 # by the consumer (do NOT use a loopback address if the consumer is remote).
 # Defaults to APP_URL when unset.
+# NOTE: the store is process-local — blobs live only on the instance that
+# created them. Behind a multi-replica load balancer WITHOUT sticky sessions a
+# consumer may hit a different instance and get a 404 (indistinguishable from an
+# expired blob). Single-host deployments are unaffected.
 # SANDBOX_PUBLIC_URL=https://docs.example.com
 # SANDBOX_TTL_MS=3600000
 # SANDBOX_MAX_BYTES=8388608