test: add comprehensive unit test suite (~85 new tests)

Add unit tests across all model and server infrastructure layers,
increasing test count from 7 to 92. Covers serde round-trips, enum
methods, defaults, config parsing, error mapping, PKCE crypto (with
RFC 7636 test vector), OAuth store, and SearXNG ranking/dedup logic.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.toml

@@ -112,6 +112,10 @@ server = [
     "dep:bytes",
 ]
 
+[dev-dependencies]
+pretty_assertions = "1.4"
+serial_test = "3.2"
+
 [[bin]]
 name = "dashboard"
 path = "bin/main.rs"