Add conditional HTTP requests with ETag and Last-Modified

[Pectics]

Context

Roadmap item imported from PLAN_REQUEST.md and PLAN_RESPONSE.md for v1.1+ — Post-v1 Backlog.

Upstream references

• [Feature]: Include clash string in UA 在默认UA中包含Clash字样 spencerwooo/mihoro#198

Upstream decision

• Ported
• Reimplemented
• Deferred
• Rejected

Reason: Deferred until source adapters and safe transactions are stable.

Scope

• Store per-profile ETag/Last-Modified metadata.
• Send conditional headers and treat 304 Not Modified as no semantic source change.

Acceptance criteria

• The scope above is implemented or documented in issue-linked PRs.
• Relevant upstream references are reviewed before implementation starts.
• Tests are added, or the PR explicitly explains why this is documentation-only.
• Documentation is updated, or the PR explicitly explains why no docs changed.
• Migration impact, security impact, and rollback behavior are evaluated.

Out of scope

• Work explicitly listed outside v1.0 in the roadmap remains deferred unless this issue is in v1.1+ — Post-v1 Backlog.
• No GitHub Project board, branch protection change, local commit, or repository file edit is part of this import pass.

Dependencies

• Depends on Add URL, local-file and existing-config source adapters #8
• Depends on Support per-profile User-Agent and authenticated HTTP headers #10

Security and rollback considerations

Apply Mihoto's safety rule for this issue: updates, migrations, service changes, TUN/DNS changes, and credential handling must be explicit, validated, auditable, and recoverable. Secrets and subscription URLs must be redacted from logs and issue artifacts.