Let users write three more CEL constructs in the filter field, each compiled to
SQL across SQLite/MySQL/Postgres:
- Scalar startsWith()/endsWith() on content/filename/mime_type (case-insensitive)
- matches() regex: PG ~, MySQL/SQLite REGEXP (Go-backed SQLite fn), validated at
compile time via cel.ValidateRegexLiterals()
- all() comprehension over tags via per-element subqueries, non-empty required
Also: contains() now escapes LIKE metacharacters (%, _, \); cross-dialect render
tests plus behavioral tests; cel-go bumped to v0.28.1; new operators surfaced in
the frontend shortcut guide.
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