feat(snippets): add snippets entity, crud, and auto-browser-per-run

- add Snippet entity with name, description, code; full CRUD backend
- add snippets pages (list, create, edit, detail) and nav entry
- add runSnippet helper in code-executor using new Function with args array
- add result param to execute() so validateCode can access exec output
- remove sessionName from steps; each run now spawns its own fresh browser
- fix waitForURL race by polling localStorage for token instead

server/src/snippet/snippet.service.ts

@@ -0,0 +1,76 @@
+import {
+  ConflictException,
+  Injectable,
+  NotFoundException,
+} from "@nestjs/common";
+import { InjectRepository } from "@nestjs/typeorm";
+import { Repository } from "typeorm";
+import { SnippetEntity } from "./snippet.entity";
+import { CreateSnippetDto } from "./dto/create-snippet.dto";
+import { UpdateSnippetDto } from "./dto/update-snippet.dto";
+import {
+  PaginationQueryDto,
+  PaginatedResult,
+} from "../common/dto/pagination.dto";
+
+export type SnippetOrderBy = "id" | "name" | "createdAt" | "updatedAt";
+
+@Injectable()
+export class SnippetService {
+  constructor(
+    @InjectRepository(SnippetEntity)
+    private readonly repo: Repository<SnippetEntity>,
+  ) {}
+
+  async create(dto: CreateSnippetDto): Promise<SnippetEntity> {
+    const existing = await this.repo.findOneBy({ name: dto.name });
+    if (existing) {
+      throw new ConflictException(`Snippet "${dto.name}" already exists`);
+    }
+    return this.repo.save(
+      this.repo.create({ ...dto, description: dto.description ?? null }),
+    );
+  }
+
+  async findAll(
+    query: PaginationQueryDto<SnippetOrderBy> = {},
+  ): Promise<PaginatedResult<SnippetEntity>> {
+    const page = query.page ?? 1;
+    const limit = query.limit ?? 50;
+    const orderBy = query.orderBy ?? "id";
+    const orderDir = query.orderDir ?? "ASC";
+    const [data, total] = await this.repo.findAndCount({
+      order: { [orderBy]: orderDir },
+      skip: (page - 1) * limit,
+      take: limit,
+    });
+    return { data, total, page, limit };
+  }
+
+  async findOne(id: number): Promise<SnippetEntity> {
+    const snippet = await this.repo.findOneBy({ id });
+    if (!snippet) throw new NotFoundException(`Snippet ${id} not found`);
+    return snippet;
+  }
+
+  async update(id: number, dto: UpdateSnippetDto): Promise<SnippetEntity> {
+    const snippet = await this.findOne(id);
+    if (dto.name && dto.name !== snippet.name) {
+      const conflict = await this.repo.findOneBy({ name: dto.name });
+      if (conflict) throw new ConflictException(`Snippet "${dto.name}" already exists`);
+    }
+    Object.assign(snippet, dto);
+    return this.repo.save(snippet);
+  }
+
+  async remove(id: number): Promise<void> {
+    await this.findOne(id);
+    await this.repo.delete(id);
+  }
+
+  /** Returns a name→code map for all snippets (used by the executor). */
+  async buildSnippetMap(): Promise<Record<string, string>> {
+    const { data } = await this.findAll({ limit: 1000 });
+    return Object.fromEntries(data.map((s) => [s.name, s.code]));
+  }
+}