Gallery search built a single `%{search}%` LIKE, so multi-word queries only matched the exact adjacent phrase ("red car" missed "a car that is red") and literal % / _ acted as SQL wildcards. Split on whitespace and require each token to match (prompt OR tags OR ai_tags), escaping LIKE wildcards per token; same escaping for the tag filter. Promote _escape_like to public escape_like_pattern() shared helper.
Part of #2123.
Tests: tests/test_gallery_search_multiword.py (5 tests); 4 fail pre-fix, 5 pass post-fix. Neighbors (session_search, gallery null-user, shims): 25 passed.
