[feat][backend] ob add manual annotation

[kidkidkid]

What type of PR is this?

Check the PR title.

• This PR title match the format: [<type>][<scope>]: <description>. For example: [fix][backend] flaky fix
• The description of this PR title is user-oriented and clear enough for others to understand.
• Add documentation if the current PR requires user awareness at the usage level.
• This PR is written in English. PRs not in English will not be reviewed.

(Optional) Translate the PR title into Chinese.

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:
zh(optional):

(Optional) Which issue(s) this PR fixes:

[codecov]

Codecov Report

❌ Patch coverage is 82.21574% with 61 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ckend/modules/observability/infra/repo/ck/spans.go	71.75%	28 Missing and 22 partials ⚠️
.../modules/observability/infra/repo/ck/annotation.go	93.13%	3 Missing and 4 partials ⚠️
...bservability/infra/repo/ck/convertor/annotation.go	93.75%	2 Missing and 2 partials ⚠️

@@            Coverage Diff             @@
##             main     #194      +/-   ##
==========================================
+ Coverage   69.92%   70.03%   +0.11%     
==========================================
  Files         610      611       +1     
  Lines       56835    57091     +256     
==========================================
+ Hits        39740    39984     +244     
+ Misses      14144    14135       -9     
- Partials     2951     2972      +21     

Flag	Coverage Δ
unittests	70.03% <82.21%> (+0.11%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...bservability/infra/repo/ck/convertor/annotation.go	93.75% <93.75%> (ø)
.../modules/observability/infra/repo/ck/annotation.go	90.00% <93.13%> (+90.00%)	⬆️
...ckend/modules/observability/infra/repo/ck/spans.go	75.44% <71.75%> (+4.37%)	⬆️

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5a30a06...bfc9168. Read the comment docs.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[CozeLoop]

Aime code review: added inline comments for correctness and robustness in spans.go (nil-map assignment; filters nil-guard; annoTable validation) and annotation.go (retry backoff). Please see the inline comments for actionable fixes.