fix(config): drop meaningless DefaultSettings UpdatedAt timestamps

[cristim]

Summary

Static DefaultSettings entries in internal/config/defaults.go stamped
UpdatedAt: time.Now() at package init -- a meaningless timestamp that could
produce false-positive "recently changed" signals versus DB-persisted user
settings.

• Replace all 28 time.Now() calls with time.Time{} (zero value).
• Add a comment documenting the zero-UpdatedAt contract.
• Rename and rewrite TestDefaultSettings_UpdatedAtSet to
  TestDefaultSettings_UpdatedAtIsZero, asserting zero value instead.
• Remove the now-unused "time" import from defaults_test.go.

Closes #1051

Note on 04-M1 overlap with #1040

Finding 04-M1 (warn on malformed env int/float values) is already addressed
by PR #1040 (fix/server-transport-config): getEnvInt, getEnvFloat, and
getTaskTimeout all log WARNING: on parse failure in that branch. This PR
covers only the residual 04-N2 item (see docs/code-review/open-questions/new-config-warn.md).

Verification

go build ./internal/config/...   # clean
go vet ./internal/config/...     # clean
go test ./internal/config/...    # 560 passed
go build ./...                   # clean

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

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Run ID: bc363e0f-d192-43c9-9b23-da31953350f8

📥 Commits

Reviewing files that changed from the base of the PR and between cbdc4be and cf44a75.

📒 Files selected for processing (2)

• internal/config/defaults.go
• internal/config/defaults_test.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/config-env-warnings

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[cristim]

Changes summary

fix(config): drop meaningless DefaultSettings UpdatedAt timestamps (finding 04-N2)

• internal/config/defaults.go: replaced 28 time.Now() calls with time.Time{} (zero value); added doc comment explaining the zero-UpdatedAt contract.
• internal/config/defaults_test.go: renamed TestDefaultSettings_UpdatedAtSet to TestDefaultSettings_UpdatedAtIsZero; updated assertion to require zero value; removed unused "time" import.

04-M1 disposition: confirmed subsumed by #1040 -- getEnvInt/getEnvFloat/getTaskTimeout in that branch already log WARNING: on parse failure. Decision logged to docs/code-review/open-questions/new-config-warn.md.

@coderabbitai review

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@cristim Sure, I'll kick off a review of this PR now.

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🧠 Learnings used

Learnt from: cristim
Repo: LeanerCloud/CUDly PR: 0
File: :0-0
Timestamp: 2026-05-05T15:44:54.463Z
Learning: In the LeanerCloud/CUDly repository, when a CodeRabbit review flags something as "outside-diff" that was already implemented in an earlier commit of the same PR, it is likely a stale-snapshot artifact where the review diff was generated against a pre-fix view. The inline comment convention `(CR pass on PR #<number> Finding N)` is used by the maintainer (cristim) to mark lines that were changed in direct response to a prior CodeRabbit review pass.

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