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

src/pages/GenerationPage.tsx

@@ -61,7 +61,7 @@ import AiTipBanner from "@/components/ai/AiTipBanner";
 import { generationService } from "@/services/generation.service";
 import { mediaService, type Avatar } from "@/services/media.service";
 import { examsService } from "@/services/exams.service";
-import { api } from "@/lib/api-client";
+import { api, describeApiError } from "@/lib/api-client";
 import { useToast } from "@/hooks/use-toast";
 import type { ExamStructure, ExamStructureConfig } from "@/types";
 
@@ -807,7 +807,12 @@ export default function GenerationPage() {
       description: `Exam #${res.exam_id} created with status "${res.status}". ${res.status === "published" ? "The exam is now live." : "Pending approval."}`,
       duration: 8000,
     }),
-    onError: (err: Error) => toast({ variant: "destructive", title: "Submit failed", description: err.message }),
+    onError: (err) =>
+      toast({
+        variant: "destructive",
+        title: "Submit failed",
+        description: describeApiError(err, "Submit failed", "module"),
+      }),
   });
 
   const anyGenerating = generatePassageMut.isPending || generateExercisesMut.isPending ||