feat: 图片支持通过标签分类

[ThaddeusJiang]

变更说明

• 将 hashtag 领域模型统一重命名为 tag，并移除兼容负担
• 图片详情页接入 Choices.js 标签输入，支持选择已有标签与创建新标签
• 统一 Choices.js 主题变量与样式，修复输入闪烁与样式不一致问题
• 上传流程改为单次 AI 调用结构化返回 caption+tags，并分别同步 Postgres/Typesense
• 客户端 re-generate-caption 调整为仅更新 caption，不再误清空 tags
• 补充 tag 功能文档与 choicejs-skill

验证

• mix test
• mix check

备注

• 本 PR 已将 worktree 变更 squash 为单一 commit

[codecov]

Codecov Report

❌ Patch coverage is 43.56846% with 136 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.12%. Comparing base (c2f4798) to head (5ed284a).
⚠️ Report is 33 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/vmemo_web/live/image_id_live.ex	0.00%	54 Missing ⚠️
lib/vmemo/memo/image.ex	6.38%	44 Missing ⚠️
lib/vmemo/ai/caption.ex	72.22%	15 Missing ⚠️
lib/vmemo/ai/ash_ai_vision.ex	0.00%	9 Missing ⚠️
lib/vmemo/memo/changes/sync_typesense.ex	0.00%	6 Missing ⚠️
lib/vmemo/memo/changes/sync_image_tags.ex	75.00%	5 Missing ⚠️
lib/vmemo_web/controllers/file_controller.ex	87.50%	1 Missing ⚠️
lib/vmemo_web/live/jobs_live.ex	0.00%	1 Missing ⚠️
lib/vmemo_web/live/tags_live/index.ex	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #199      +/-   ##
==========================================
+ Coverage   36.18%   38.12%   +1.93%     
==========================================
  Files         119      123       +4     
  Lines        5441     5986     +545     
==========================================
+ Hits         1969     2282     +313     
- Misses       3472     3704     +232     

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