feat: hide entry content from list, add GET /api/entries/:id/content endpoint

Content is no longer returned in the entries list. A separate endpoint
returns plain text content for clipboard copying on demand.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "backend"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+axum = "0.8"
+sqlx = { version = "0.8", features = ["runtime-tokio", "postgres", "chrono"] }
+tokio = { version = "1", features = ["full"] }
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+tower-http = { version = "0.6", features = ["cors"] }
+chrono = { version = "0.4", features = ["serde"] }