Enable username autocomplete in thread titles

[Oaphi]

Noticed when making the update post: even though we now properly render pings in thread titles, we don't offer any autocomplete - this PR addresses this shortcoming:

ssr-2025-08-02_14.48.14.mp4

Autocomplete will also become available when renaming threads:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.50%. Comparing base (40d00c2) to head (15942f1).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

Components	Coverage Δ
controllers	68.67% <ø> (-0.04%)	⬇️
helpers	79.01% <ø> (ø)
jobs	48.57% <ø> (ø)
models	85.86% <ø> (ø)

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