Add endpoint to generate a flow start preview

[rowanseymour]

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[codecov]

Codecov Report

Merging #619 (57a1381) into main (f03cee1) will increase coverage by 0.12%.
The diff coverage is 86.95%.

@@            Coverage Diff             @@
##             main     #619      +/-   ##
==========================================
+ Coverage   62.76%   62.88%   +0.12%     
==========================================
  Files         127      128       +1     
  Lines       12772    12840      +68     
==========================================
+ Hits         8016     8075      +59     
- Misses       3884     3890       +6     
- Partials      872      875       +3     

Impacted Files	Coverage Δ
web/flow/start.go	86.76% <86.76%> (ø)
core/models/search.go	70.33% <100.00%> (ø)

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