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.

crates/bootstrap/Cargo.toml

@@ -12,3 +12,6 @@ http-api.workspace = true
 http-json.workspace = true
 tokio.workspace = true
 anyhow.workspace = true
+tracing.workspace = true
+tracing-subscriber.workspace = true
+dotenvy.workspace = true