feat: add AfterScanner hook for post-scan field hydration

[klaidliadon]

Summary

• Adds AfterScanner interface: structs implementing AfterScan() error get called automatically after scan in Table methods
• Hook points: Get, List, ListPaged, Iter — zero reflection, pure generic type assertions
• Zero cost for types that don't implement the interface

Refs #43

Test plan

• Get with AfterScanner — verifies hook runs and modifies fields
• List with AfterScanner — verifies hook runs on each element
• Get without AfterScanner — verifies no-op (existing behavior preserved)
• Error propagation — Get and List surface AfterScan errors
• Full existing test suite passes

[klaidliadon]

Closing per Vojtech's feedback — hooks make pgkit too ORM-like. Moving the presentation logic to the OMSX RPC layer instead.