fix(ui): actionable 413/504/401 toasts + VPS nginx deploy template

Production QA reported "Submit failed 413" on /generation and plain
"Upload failed" (no detail) on resource upload. Root cause is the
outer reverse-proxy nginx on the VPS — still on default
`client_max_body_size 1m` — rejecting requests before they reach
Odoo. The inner docker-nginx and Odoo limits were already raised in
earlier commits; only the VPS proxy was left unpatched.

UI side
- Add `describeApiError(err, fallback, context)` helper in
  lib/api-client.ts. Maps common HTTP codes to human-readable,
  actionable toast descriptions (413 → "ask ops to raise nginx
  client_max_body_size", 504 → "server took too long, retry", 502
  → "Odoo is restarting", 401 → "sign in again", 4xx/5xx → server
  error body + status code).
- Use it in GenerationPage submit, ResourceManager upload,
  TeacherLibrary upload, CustomExamCreate save/publish. Replaces
  four slightly-different ad-hoc mappings with one source of truth.

Deploy side
- Add deploy/nginx-vps.conf.example: full TLS reverse-proxy template
  with the body/timeout knobs that must match Odoo's limit_request
  (128 MB) and limit_time_real (900 s).
- Add docs/DEPLOY_413_504_FIX.md: step-by-step remediation guide for
  the team lead covering the 3 layers that can block large bodies
  (outer nginx, Cloudflare, Odoo worker) and how to verify each.

Made-with: Cursor

frontend/src/pages/teacher/TeacherLibrary.tsx

@@ -15,6 +15,7 @@ import {
 } from "lucide-react";
 import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
 import { resourcesService } from "@/services/resources.service";
+import { describeApiError } from "@/lib/api-client";
 import { coursewareService } from "@/services/courseware.service";
 import { taxonomyService } from "@/services/taxonomy.service";
 import { useToast } from "@/hooks/use-toast";
@@ -116,14 +117,12 @@ export default function TeacherLibrary() {
       setUploadTopicIds([]);
       setUploadObjectiveIds([]);
     },
-    onError: (err: unknown) => {
-      const e = err as { status?: number; message?: string };
-      const status = e?.status;
-      let description = e?.message || "Upload failed";
-      if (status === 413) description = "File is too large for the server (HTTP 413). Try a smaller file or ask an admin to raise the upload limit.";
-      else if (status === 401) description = "Session expired — please sign in again.";
-      else if (status === 504) description = "Server timed out while saving the file (HTTP 504). Try a smaller file or retry.";
-      toast({ title: "Upload failed", description, variant: "destructive" });
+    onError: (err) => {
+      toast({
+        title: "Upload failed",
+        description: describeApiError(err, "Upload failed", "file"),
+        variant: "destructive",
+      });
     },
   });