docs: add .env.example as the deployment template

`cp .env.example .env` is now the documented starting point: the tracked
template carries every variable (grouped required / sites / bot behaviour /
network / webhook / reverse proxy) with the defaults the code would use
anyway, and the compose comment plus both READMEs point at it. The proxy note
is spelled out where it matters — teloxide panics on a blank `TELOXIDE_PROXY`,
and inside a container the proxy host must be `host.docker.internal`.

Two follow-ups the template exposed:

- `RUST_LOG=` (present but blank, which `.env` makes easy) silenced the log
  again: "unset" was handled, "empty" was not. A blank value now falls back to
  the same default. Verified: blank and unset both produce the full startup
  sequence.
- `TWITTER_AUTH_TOKEN` was in the README prose but missing from the env table
  (both languages).

Verified: `docker compose --env-file .env.example config -q` resolves, and a
script comparing the compose's `${VAR}` references against the template's keys
finds none missing.

crates/xmedia-bot/src/main.rs

@@ -97,12 +97,15 @@ async fn main() {
     // with no logs; and at `debug` the HTTP client's own lines (hyper_util,
     // reqwest) outnumbered the bot's by two to one. The timed builder adds
     // the timestamp the plain `init` omitted, so a line can be compared with
-    // a user's report. An explicit RUST_LOG still wins outright.
+    // a user's report. An explicit RUST_LOG still wins outright — but a blank
+    // one (`RUST_LOG=` in `.env`, which is not "unset") must not silence the
+    // log the way its absence used to.
+    let filter = std::env::var("RUST_LOG")
+        .ok()
+        .filter(|value| !value.trim().is_empty())
+        .unwrap_or_else(|| "info,hyper_util=warn,reqwest=warn".to_string());
     pretty_env_logger::formatted_timed_builder()
-        .parse_filters(
-            &std::env::var("RUST_LOG")
-                .unwrap_or_else(|_| "info,hyper_util=warn,reqwest=warn".to_string()),
-        )
+        .parse_filters(&filter)
         .init();
     log::info!("Starting bot");