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/adapters/tcp-server/Cargo.toml

@@ -9,3 +9,4 @@ protocol.workspace = true
 tokio.workspace = true
 postcard.workspace = true
 thiserror.workspace = true
+tracing.workspace = true