feat(prisma): add server-only caching for problems queries

- Add 'server-only' import to enforce server-side usage
- Implement cached problem queries with logging:
  - Add getProblems/getCachedProblems for all problems
  - Add getProblem/getCachedProblem for single problem by ID
- Use React cache and Next.js unstable_cache with tags
- Add detailed logging with timing metrics

src/lib/prisma.ts

@@ -1,5 +1,12 @@
+import "server-only";
+
+import { cache } from "react";
+import { logger } from "@/lib/logger";
+import { unstable_cache } from "next/cache";
 import { PrismaClient } from "@/generated/client";
 
+const log = logger.child({ module: "prisma" });
+
 const globalForPrisma = globalThis as unknown as { prisma: PrismaClient };
 
 const prisma = globalForPrisma.prisma || new PrismaClient();
@@ -7,3 +14,99 @@ const prisma = globalForPrisma.prisma || new PrismaClient();
 if (process.env.NODE_ENV !== "production") globalForPrisma.prisma = prisma;
 
 export default prisma;
+
+const getProblems = async () => {
+  const startTime = Date.now();
+  log.debug("Fetching all problems from database");
+  try {
+    const problems = await prisma.problem.findMany();
+    log.debug(
+      { count: problems.length, durationMs: Date.now() - startTime },
+      "Fetched problems successfully"
+    );
+    return problems;
+  } catch (error) {
+    log.error(
+      { durationMs: Date.now() - startTime, error },
+      "Failed to fetch problems"
+    );
+    throw error;
+  }
+};
+
+export const getCachedProblems = cache(
+  unstable_cache(
+    async () => {
+      const startTime = Date.now();
+      log.debug("Calling getProblemsCached (expect cache hit if warmed)");
+      try {
+        const result = await getProblems();
+        log.info(
+          { durationMs: Date.now() - startTime },
+          "getProblemsCached finished"
+        );
+        return result;
+      } catch (error) {
+        log.error(
+          { durationMs: Date.now() - startTime, error },
+          "getProblemsCached failed"
+        );
+        throw error;
+      }
+    },
+    ["getProblems"],
+    {
+      tags: ["problems"],
+    }
+  )
+);
+
+const getProblem = async (id: string) => {
+  const startTime = Date.now();
+  log.debug({ id }, "Fetching single problem");
+  try {
+    const problem = await prisma.problem.findUnique({ where: { id } });
+    if (problem) {
+      log.debug({ id, durationMs: Date.now() - startTime }, "Problem found");
+    } else {
+      log.warn({ id, durationMs: Date.now() - startTime }, "Problem not found");
+    }
+    return problem;
+  } catch (error) {
+    log.error(
+      { id, durationMs: Date.now() - startTime, error },
+      "Failed to fetch problem"
+    );
+    throw error;
+  }
+};
+
+export const getCachedProblem = cache((id: string) =>
+  unstable_cache(
+    async () => {
+      const startTime = Date.now();
+      log.debug(
+        { id },
+        "Calling getProblemCached (expect cache hit if warmed)"
+      );
+      try {
+        const result = await getProblem(id);
+        log.info(
+          { id, durationMs: Date.now() - startTime },
+          "getProblemCached finished"
+        );
+        return result;
+      } catch (error) {
+        log.error(
+          { id, durationMs: Date.now() - startTime, error },
+          "getProblemCached failed"
+        );
+        throw error;
+      }
+    },
+    ["getProblem", id],
+    {
+      tags: [`problem-${id}`],
+    }
+  )()
+);