chore: Replace 'db' with 'sa' for SQLAlchemy compatibility

[asukaminato0721]

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Summary

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Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods

[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	38.61%	38.61%	0.00%
Strict coverage	35.74%	35.74%	0.00%
Typed symbols	18,173	18,173	0
Untyped symbols	31,220	31,220	0
Modules	2516	2516	0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.43%. Comparing base (3c7d673) to head (ec52eae).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #35373   +/-   ##
=======================================
  Coverage   85.43%   85.43%           
=======================================
  Files        4431     4431           
  Lines      206713   206714    +1     
  Branches    38470    38470           
=======================================
+ Hits       176612   176613    +1     
  Misses      26992    26992           
  Partials     3109     3109           

Flag	Coverage Δ
api	84.78% <100.00%> (+<0.01%)	⬆️

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[Copilot]

Pull request overview

Updates workflow comment ORM models to use sqlalchemy as sa types/constructs instead of db.*, aligning these models with the broader SQLAlchemy-first patterns used across api/models.

Changes:

• Introduce import sqlalchemy as sa in api/models/comment.py.
• Replace db.* column/constraint/type usages with sa.* equivalents (e.g., PrimaryKeyConstraint, Text, DateTime, Boolean, text).

Comments suppressed due to low confidence (1)

api/models/comment.py:203

• These FK columns still use db.ForeignKey(...) while this PR is replacing db.* constructs with sqlalchemy as sa. Consider switching both to sa.ForeignKey(...) (or sqlalchemy.ForeignKey) for consistency and to avoid coupling schema definitions to the Flask-SQLAlchemy db instance.

    comment_id: Mapped[str] = mapped_column(
        StringUUID, db.ForeignKey("workflow_comments.id", ondelete="CASCADE"), nullable=False
    )
    reply_id: Mapped[str | None] = mapped_column(
        StringUUID, db.ForeignKey("workflow_comment_replies.id", ondelete="CASCADE"), nullable=True
    )

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