feat(moment): 動態打分改用專用模型（fallback 短內容，零停機）

[mashbean]

背景

共用短內容模型對動態(moment) ~90% 華語誤殺(把散文/詩/貓咖/政治時評打 1.0)。已重訓專用動態模型(label-fix,e5+head),699 筆非訓練集 held-out(subagent 真標籤)實測:

門檻	舊共用短內容模型	新專用動態模型
@0.5	誤殺95.5%/召回69%	誤殺2.5%/召回97.8%
@0.7	誤殺93.9%/召回67%	誤殺0.8%/召回92.7%

改動（比照 #4838 留言拆專用）

• environment.ts：加 momentSpamDetectionApiUrl（MATTERS_MOMENT_SPAM_DETECTION_API_URL）
• momentService.detectSpam：改用專用 endpoint,未設定時 fallback 短內容模型 → 零停機 opt-in
• .env.example 補上

啟用

ops 設 MATTERS_MOMENT_SPAM_DETECTION_API_URL 指向已部署的 prod 動態 endpoint(stack moment-spam-model-service)即啟用。未設定則行為不變。

注意:動態實際處置仍以 ring 行為訊號為主(影子週 06-24 報告後決定);本模型分數讓動態打分變可信、作為輔助訊號。

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.77%. Comparing base (915098c) to head (463e728).
⚠️ Report is 41 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4860      +/-   ##
===========================================
- Coverage    72.93%   72.77%   -0.17%     
===========================================
  Files         1068     1071       +3     
  Lines        21541    21306     -235     
  Branches      4802     4657     -145     
===========================================
- Hits         15712    15505     -207     
+ Misses        5347     5324      -23     
+ Partials       482      477       -5     

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