fix(preview): preserve comment DOM anchors across fetch + re-share

[tomasz-tomczyk]

Summary

Preview comments are DOM pins, but dom_anchor was being dropped on two round-trip legs (found in the pre-release audit of v0.9.0..main):

• Export leg — output.ex serialize_comment_for_export/1 (the crit fetch export at GET /api/export/:token/comments) omitted dom_anchor, so the CLI couldn't read anchors back. Now aligned with the live serializer Reviews.serialize_comment/1.
• Persist leg — reviews.ex replace_comments/3 hand-built its attrs map without dom_anchor, so re-sharing/upserting a review stripped every anchor. Now carries it forward (prefer payload, else the existing row matched by external_id, mirroring the user_id attribution carry-forward).

Also removes a dead stats: Crit.Statistics.totals() assign from the homepage controller — no page_html template consumes @stats since the marketing revamp removed the stats band; it was an unused DB aggregate on every homepage load.

Review

• Code review: passed (fresh expert review; fix lines reverted to confirm tests are non-vacuous)
• Parity audit: N/A (backend)

Test plan

• mix precommit (compile --warnings-as-errors, format, sobelow, tests): 1018 tests, 0 failures
• New tests: dom_anchor survives re-share (payload + carry-forward via external_id); export JSON includes dom_anchor
• See also: fix: dedup heading-anchor IDs and add controls to reduced-motion demo videos #245 (heading-anchor + reduced-motion fixes from the same audit)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.43%. Comparing base (21a90e5) to head (6c8e53d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #244      +/-   ##
==========================================
+ Coverage   82.42%   82.43%   +0.01%     
==========================================
  Files          89       89              
  Lines        3089     3091       +2     
==========================================
+ Hits         2546     2548       +2     
  Misses        543      543              

Flag	Coverage Δ
unit	82.43% <100.00%> (+0.01%)	⬆️

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