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.

.env.example

@@ -0,0 +1,8 @@
+# K-Frame Server Configuration
+KFRAME_DATABASE_URL=sqlite:kframe.db?mode=rwc
+KFRAME_TCP_ADDR=0.0.0.0:2699
+KFRAME_HTTP_ADDR=0.0.0.0:3000
+KFRAME_POLL_INTERVAL_SECS=5
+
+# Logging (tracing-subscriber)
+RUST_LOG=info,sqlx=warn