Streaming support for conversation history with SSE (#167)

* Streaming support with SSE

The main focus was on Gemini-CLI token streaming, which uses the standard JSON-Patch format to stream real-time updates to the frontend visa SSE.

There is also a default database-backed SSE implementation which covers the remaining executors like Claude-code.

* minor refactorings

backend/Cargo.toml

@@ -50,9 +50,13 @@ reqwest = { version = "0.11", features = ["json"] }
 strip-ansi-escapes = "0.2.1"
 urlencoding = "2.1.3"
 lazy_static = "1.4"
+futures-util = "0.3"
+async-stream = "0.3"
+json-patch = "2.0"
 
 [dev-dependencies]
 tempfile = "3.8"
+tower = { version = "0.4", features = ["util"] }
 
 [build-dependencies]
 dotenv = "0.15"