feat(sandbox): in-RAM blob sandbox for out-of-band page transfer (#243)

Add an ephemeral, process-local blob store so the in-app agent (and the
embedded MCP) can hand a large page document and its images to an external
consumer WITHOUT routing the bytes through the model context or Docmost auth.

- SandboxStore (@Injectable singleton): Map<uuid,{buf,mime,sha256,expiresAt}>
  in RAM only. put() picks a per-blob cap by mime (image vs doc), enforces a
  total-bytes RAM guard with oldest-first eviction, and stamps a TTL; get()
  lazily expires. sha256 computed at put() doubles as the strong ETag. An
  unref'd sweep interval clears expired entries and is cleared on destroy.
- GET /api/sb/:uuid anonymous controller: serves raw bytes with Content-Type,
  Content-Length and ETag=sha256; 404 on missing/expired/non-UUID (anti-
  traversal), 304 on a matching If-None-Match. No tokens, no 401 — the
  capability is the unguessable UUID + short TTL + TLS. Auth-exempt the same
  way as /api/files/public (no JwtAuthGuard) plus an /api/sb entry in main.ts's
  workspace-resolution preHandler so a remote consumer with no workspace host
  is not rejected.
- stash_page tool in both layers (MCP resource_link + in-app {uri,size,sha256,
  images}). client.stashPage serializes the get_page_json shape, mirrors every
  INTERNAL file/image src (type-agnostic, covers drawio/excalidraw/video/file)
  into the sandbox under Docmost auth and rewrites src to the sandbox URL;
  external http(s) srcs are left untouched; dedup by src; a failed image fetch
  is counted, never aborts the doc.
- SANDBOX_PUBLIC_URL / SANDBOX_TTL_MS / SANDBOX_MAX_BYTES /
  SANDBOX_MAX_IMAGE_BYTES / SANDBOX_MAX_TOTAL_BYTES wired through the
  environment service + validation + .env.example.
- SandboxModule (@Global) provides the shared store to the controller,
  McpService and AiChatToolsService (same instance for put and get).

Tests: SandboxStore (round-trip, sha256, TTL lazy + sweep, caps, eviction),
SandboxController (200+ETag+CT+CL, 404 missing/expired/non-UUID, 304), and a
mock-HTTP stashPage test (mirror+rewrite internal, keep external, dedup, failed
image counted, returns only a link). Interoperates with the vvzvlad/habr-mcp
consumer's anonymous-GET + sha256-ETag + resource_link contract.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/integrations/environment/environment.validation.ts

@@ -2,6 +2,7 @@ import {
   IsIn,
   IsNotEmpty,
   IsNotIn,
+  IsNumberString,
   IsOptional,
   IsString,
   IsUrl,
@@ -170,6 +171,35 @@ export class EnvironmentVariables {
     },
   )
   CLICKHOUSE_URL: string;
+
+  // --- Blob sandbox (in-RAM ephemeral blob transfer; see SandboxModule) ---
+
+  @IsOptional()
+  @ValidateIf((obj) => obj.SANDBOX_PUBLIC_URL != '' && obj.SANDBOX_PUBLIC_URL != null)
+  @IsUrl(
+    { protocols: ['http', 'https'], require_tld: false },
+    {
+      message:
+        'SANDBOX_PUBLIC_URL must be a valid http(s) URL reachable by the external blob consumer',
+    },
+  )
+  SANDBOX_PUBLIC_URL: string;
+
+  @IsOptional()
+  @IsNumberString({}, { message: 'SANDBOX_TTL_MS must be an integer (milliseconds)' })
+  SANDBOX_TTL_MS: string;
+
+  @IsOptional()
+  @IsNumberString({}, { message: 'SANDBOX_MAX_BYTES must be an integer (bytes)' })
+  SANDBOX_MAX_BYTES: string;
+
+  @IsOptional()
+  @IsNumberString({}, { message: 'SANDBOX_MAX_IMAGE_BYTES must be an integer (bytes)' })
+  SANDBOX_MAX_IMAGE_BYTES: string;
+
+  @IsOptional()
+  @IsNumberString({}, { message: 'SANDBOX_MAX_TOTAL_BYTES must be an integer (bytes)' })
+  SANDBOX_MAX_TOTAL_BYTES: string;
 }
 
 export function validate(config: Record<string, any>) {