Add db, repo and config package unit test coverage

[yulmwu]

Refs: #4

> go test -v -race -cover -count=1 ./internal/db ./internal/repo
2026/01/29 11:58:24 github.com/testcontainers/testcontainers-go - Connected to docker: 
  Server Version: 28.2.2
  API Version: 1.50
  Operating System: Docker Desktop
  Total Memory: 3919 MB
  Labels:
    com.docker.desktop.address=unix:///Users/user/Library/Containers/com.docker.docker/Data/docker-cli.sock
  Testcontainers for Go Version: v0.40.0
  Resolved Docker Host: unix:///var/run/docker.sock
  Resolved Docker Socket Path: /var/run/docker.sock
  Test SessionID: f045ce4866d7bf91ddd769c944e698a278b2fea035acfb8644aa2c024b405c41
  Test ProcessID: 4fa51f5a-53c0-41d4-a7c5-ea1ee140542f
2026/01/29 11:58:24 🐳 Creating container for image postgres:16-alpine
2026/01/29 11:58:24 🐳 Creating container for image testcontainers/ryuk:0.13.0
2026/01/29 11:58:24 ✅ Container created: 7ff4181fa9fa
2026/01/29 11:58:24 🐳 Starting container: 7ff4181fa9fa
2026/01/29 11:58:24 ✅ Container started: 7ff4181fa9fa
2026/01/29 11:58:24 ⏳ Waiting for container id 7ff4181fa9fa image: testcontainers/ryuk:0.13.0. Waiting for: port 8080/tcp to be listening
2026/01/29 11:58:24 Shell not found in container
2026/01/29 11:58:24 🔔 Container is ready: 7ff4181fa9fa
2026/01/29 11:58:24 ✅ Container created: f618361d543e
2026/01/29 11:58:24 🐳 Starting container: f618361d543e
2026/01/29 11:58:25 ✅ Container started: f618361d543e
2026/01/29 11:58:25 ⏳ Waiting for container id f618361d543e image: postgres:16-alpine. Waiting for: port 5432/tcp to be listening
2026/01/29 11:58:26 🔔 Container is ready: f618361d543e
=== RUN   TestIsUniqueViolation
--- PASS: TestIsUniqueViolation (0.00s)
=== RUN   TestNewAndAutoMigrate
--- PASS: TestNewAndAutoMigrate (0.06s)
=== RUN   TestEnsureColumnsAndIndexes
--- PASS: TestEnsureColumnsAndIndexes (0.01s)
=== RUN   TestCreateTables
--- PASS: TestCreateTables (0.00s)
PASS
coverage: 79.5% of statements
2026/01/29 11:58:26 🐳 Stopping container: f618361d543e
2026/01/29 11:58:26 ✅ Container stopped: f618361d543e
2026/01/29 11:58:26 🐳 Terminating container: f618361d543e
2026/01/29 11:58:26 🚫 Container terminated: f618361d543e
ok      smctf/internal/db       3.700s  coverage: 79.5% of statements
2026/01/29 11:58:24 github.com/testcontainers/testcontainers-go - Connected to docker: 
  Server Version: 28.2.2
  API Version: 1.50
  Operating System: Docker Desktop
  Total Memory: 3919 MB
  Labels:
    com.docker.desktop.address=unix:///Users/user/Library/Containers/com.docker.docker/Data/docker-cli.sock
  Testcontainers for Go Version: v0.40.0
  Resolved Docker Host: unix:///var/run/docker.sock
  Resolved Docker Socket Path: /var/run/docker.sock
  Test SessionID: f045ce4866d7bf91ddd769c944e698a278b2fea035acfb8644aa2c024b405c41
  Test ProcessID: 5f1e912c-9a02-4238-9d74-faef2e470081
2026/01/29 11:58:24 🐳 Creating container for image postgres:16-alpine
2026/01/29 11:58:24 ⏳ Waiting for Reaper "7ff4181f" to be ready
2026/01/29 11:58:24 🔥 Reaper obtained from Docker for this test session 7ff4181f
2026/01/29 11:58:24 ✅ Container created: 71951f01d608
2026/01/29 11:58:24 🐳 Starting container: 71951f01d608
2026/01/29 11:58:25 ✅ Container started: 71951f01d608
2026/01/29 11:58:25 ⏳ Waiting for container id 71951f01d608 image: postgres:16-alpine. Waiting for: port 5432/tcp to be listening
2026/01/29 11:58:26 🔔 Container is ready: 71951f01d608
=== RUN   TestChallengeRepoCRUD
--- PASS: TestChallengeRepoCRUD (0.02s)
=== RUN   TestChallengeRepoNotFound
--- PASS: TestChallengeRepoNotFound (0.02s)
=== RUN   TestWrapError
--- PASS: TestWrapError (0.00s)
=== RUN   TestMapNotFound
--- PASS: TestMapNotFound (0.00s)
=== RUN   TestWrapNotFound
--- PASS: TestWrapNotFound (0.00s)
=== RUN   TestRegistrationKeyRepoCRUD
--- PASS: TestRegistrationKeyRepoCRUD (0.05s)
=== RUN   TestRegistrationKeyRepoNotFound
--- PASS: TestRegistrationKeyRepoNotFound (0.01s)
=== RUN   TestSubmissionRepoCreateAndHasCorrect
--- PASS: TestSubmissionRepoCreateAndHasCorrect (0.03s)
=== RUN   TestSubmissionRepoSolvedChallenges
--- PASS: TestSubmissionRepoSolvedChallenges (0.04s)
=== RUN   TestSubmissionRepoSolvedChallengesEmpty
--- PASS: TestSubmissionRepoSolvedChallengesEmpty (0.02s)
=== RUN   TestUserRepoCRUD
--- PASS: TestUserRepoCRUD (0.03s)
=== RUN   TestUserRepoNotFound
--- PASS: TestUserRepoNotFound (0.01s)
=== RUN   TestUserRepoLeaderboardAndTimeline
--- PASS: TestUserRepoLeaderboardAndTimeline (0.05s)
=== RUN   TestUserRepoTimelineNoSince
--- PASS: TestUserRepoTimelineNoSince (0.03s)
=== RUN   TestUserRepoTimelineOrdering
--- PASS: TestUserRepoTimelineOrdering (0.03s)
=== RUN   TestUserRepoListOrdering
--- PASS: TestUserRepoListOrdering (0.04s)
=== RUN   TestUserRepoLeaderboardTieBreak
--- PASS: TestUserRepoLeaderboardTieBreak (0.04s)
=== RUN   TestUserRepoTimelineIncludesUsername
--- PASS: TestUserRepoTimelineIncludesUsername (0.03s)
=== RUN   TestUserRepoCreateDuplicateEmail
[bun]  11:58:26.612   INSERT                  358µs  INSERT INTO "users" ("id", "email", "username", "password_hash", "role", "created_at", "updated_at") VALUES (DEFAULT, 'dup@example.com', 'dup2', 'hash', 'user', '2026-01-29 02:58:26.612486+00:00', '2026-01-29 02:58:26.612486+00:00') RETURNING "id"        pgdriver.Error: ERROR: duplicate key value violates unique constraint "users_email_key" (SQLSTATE=23505) 
--- PASS: TestUserRepoCreateDuplicateEmail (0.01s)
PASS
coverage: 83.0% of statements
2026/01/29 11:58:26 🐳 Stopping container: 71951f01d608
2026/01/29 11:58:26 ✅ Container stopped: 71951f01d608
2026/01/29 11:58:26 🐳 Terminating container: 71951f01d608
2026/01/29 11:58:26 🚫 Container terminated: 71951f01d608
ok      smctf/internal/repo     4.155s  coverage: 83.0% of statements

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[yulmwu]

• Instead of rebasing the branches, merge them so that the PR Add unit test and/or integration test coverage #5 commit history is visible.
• http package has relatively low coverage, but I don't think it needs testing, so I'll ignore it.
• Codecov's coverage exceeded 74.22%. We achieved our target of 70% (Increase unit test and/or integration test coverage #4). We will squash and close branch Increase unit test and/or integration test coverage #4.
• Refer: https://app.codecov.io/github/nullforu/smctf