Phase 2: Chat & AI Core — Claude API streaming, chat UI, admin context

Backend:
- Chat, Message, ContextFile models + Alembic migration
- Chat CRUD with per-user limit enforcement (max_chats)
- SSE streaming endpoint: saves user message, streams Claude response,
  saves assistant message with token usage metadata
- Context assembly: primary context file + conversation history
- Admin context CRUD (GET/PUT with version tracking)
- Anthropic SDK integration with async streaming
- Chat ownership isolation (users can't access each other's chats)

Frontend:
- Chat page with sidebar chat list + main chat window
- Real-time SSE streaming via fetch + ReadableStream
- Message bubbles with Markdown rendering (react-markdown)
- Auto-growing message input (Enter to send, Shift+Enter newline)
- Zustand chat store for streaming state management
- Admin primary context editor with unsaved changes warning
- Updated routing: /chat, /chat/:chatId, /admin/context
- Enabled Chat and Admin sidebar navigation
- English + Russian translations for all new UI

Infrastructure:
- nginx: disabled proxy buffering for SSE support
- Added ANTHROPIC_API_KEY and CLAUDE_MODEL to config

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/admin/context-editor.tsx

@@ -0,0 +1,92 @@
+import { useState, useEffect } from "react";
+import { useTranslation } from "react-i18next";
+import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
+import { getPrimaryContext, updatePrimaryContext } from "@/api/admin";
+import { Save } from "lucide-react";
+
+export function ContextEditor() {
+  const { t } = useTranslation();
+  const queryClient = useQueryClient();
+  const [content, setContent] = useState("");
+  const [hasChanges, setHasChanges] = useState(false);
+
+  const query = useQuery({
+    queryKey: ["admin-context"],
+    queryFn: getPrimaryContext,
+  });
+
+  useEffect(() => {
+    if (query.data) {
+      setContent(query.data.content);
+      setHasChanges(false);
+    }
+  }, [query.data]);
+
+  const mutation = useMutation({
+    mutationFn: (text: string) => updatePrimaryContext(text),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ["admin-context"] });
+      setHasChanges(false);
+    },
+  });
+
+  const handleChange = (value: string) => {
+    setContent(value);
+    setHasChanges(true);
+  };
+
+  const handleSave = () => {
+    mutation.mutate(content);
+  };
+
+  // Warn on unsaved changes
+  useEffect(() => {
+    const handler = (e: BeforeUnloadEvent) => {
+      if (hasChanges) e.preventDefault();
+    };
+    window.addEventListener("beforeunload", handler);
+    return () => window.removeEventListener("beforeunload", handler);
+  }, [hasChanges]);
+
+  return (
+    <div className="space-y-4">
+      <div className="flex items-center justify-between">
+        <div>
+          <h1 className="text-2xl font-semibold tracking-tight">
+            {t("admin.context_editor")}
+          </h1>
+          {query.data && (
+            <p className="text-sm text-muted-foreground">
+              {t("admin.version")}: {query.data.version}
+            </p>
+          )}
+        </div>
+        <div className="flex items-center gap-2">
+          {hasChanges && (
+            <span className="text-sm text-amber-500">{t("admin.unsaved_changes")}</span>
+          )}
+          <button
+            onClick={handleSave}
+            disabled={!hasChanges || mutation.isPending}
+            className="inline-flex h-9 items-center gap-2 rounded-md bg-primary px-4 text-sm font-medium text-primary-foreground hover:bg-primary/90 disabled:opacity-50 transition-colors"
+          >
+            <Save className="h-4 w-4" />
+            {mutation.isPending ? t("common.loading") : t("admin.save")}
+          </button>
+        </div>
+      </div>
+
+      <textarea
+        value={content}
+        onChange={(e) => handleChange(e.target.value)}
+        rows={20}
+        className="w-full resize-y rounded-lg border border-input bg-background px-4 py-3 text-sm font-mono ring-offset-background placeholder:text-muted-foreground focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring"
+        placeholder={t("admin.context_placeholder")}
+      />
+
+      <p className="text-xs text-muted-foreground text-right">
+        {content.length.toLocaleString()} {t("admin.characters")}
+      </p>
+    </div>
+  );
+}