fix(search): gallery library per-word search with LIKE escaping (#6584)

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.

src/session_search.py

@@ -57,10 +57,19 @@ def _message_to_context(msg: DBChatMessage) -> dict[str, Any]:
     }
 
 
-def _escape_like(value: str) -> str:
+def escape_like_pattern(value: str) -> str:
+    """Escape SQL LIKE wildcards (``%``, ``_``) and the escape char itself.
+
+    Public shared helper for building ``ilike(f"%{...}%", escape="\\\\")``
+    patterns from user input. Must escape backslash first.
+    """
     return value.replace("\\", "\\\\").replace("%", "\\%").replace("_", "\\_")
 
 
+def _escape_like(value: str) -> str:
+    return escape_like_pattern(value)
+
+
 def _snippet(content: str, query: str, radius: int = 60) -> str:
     content = content or ""
     query = query or ""