add tracing, env config, dotenvy

bootstrap: tracing-subscriber with RUST_LOG env filter, ServerConfig
from env vars (KFRAME_DATABASE_URL, KFRAME_TCP_ADDR, etc.), dotenvy
for .env file loading. Replaced all println with tracing macros.

tcp-server: replaced println with tracing::info/warn.

Added .env.example and .gitignore for db files.

Cargo.toml

@@ -41,6 +41,9 @@ tower-http = { version = "0.6", features = ["cors"] }
 api-types = { path = "crates/api-types" }
 thiserror = "2.0"
 anyhow = "1.0"
+tracing = "0.1"
+tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+dotenvy = "0.15"
 serde = { version = "1.0", default-features = false, features = ["derive", "alloc"] }
 serde_json = "1.0"
 sqlx = { version = "0.8", features = ["runtime-tokio-rustls", "sqlite"] }