feat(sidecar): make the sidecar installable — CLI, --print-config, anchored data paths

Phase 0.2 of the installer plan (docs/installer/PLAN.md §5). The installer has to
drive this binary non-interactively, and today it cannot: the auth token is only
readable by scraping the startup log, the config path can only be named through an
environment variable, and a relative db path follows the process working directory —
which a service manager, not the operator, chooses.

- Add a four-flag CLI (cli.rs): --print-config, --config <PATH>, --version, --help.
  Hand-rolled; an argument-parsing dependency would be larger than the code it
  replaced. An unrecognized flag exits 2 rather than starting a sidecar that is not
  the one that was asked for.
- --print-config resolves the configuration exactly as a normal start does —
  including writing a missing config file and generating a blank auth token — and
  prints it as JSON on stdout: versions, protocol, both bind addresses, ws path,
  resolved db path, and the token. config_created / token_generated let a re-run
  tell "read an existing install" from "provisioned a new one". The log subscriber
  is deliberately not started in this mode, so the document is the whole output.
- Anchor a relative [store].path to the config file's directory instead of the CWD,
  and report resolved absolute paths. A unit pinning UOLINK_CONFIG now keeps its
  database beside its config rather than in %SystemRoot%\System32 or a VirtualStore
  redirect. Development is unaffected: under cargo run the two directories are the
  same. :memory: and file: URIs are left alone.
- Hand the db path to sqlx as a filesystem path instead of formatting it into a
  sqlite:// URL, which percent-decodes it and splits it on '?'. An installed path
  containing %20 previously opened a different file; verified it now does not.
- Create the config's and the database's parent directories when missing, so a
  service can name /var/lib/runicgateway on a host where nothing made it yet.
- 22 unit tests covering argument parsing, path anchoring, token persistence, the
  generated config template, and the --print-config document.

No protocol change: PROTOCOL_VERSION stays 3.

Co-Authored-By: Claude <noreply@anthropic.com>

sidecar/src/main.rs

@@ -3,6 +3,7 @@
 //! Terminates the loopback link to the ServUO shard and exposes a website-facing HTTP surface. So
 //! far: the shard link (bidirectional) and a WebSocket live feed. REST queries and SQLite come next.
 
+mod cli;
 mod config;
 mod rpc;
 mod shard;
@@ -33,13 +34,48 @@ pub const PROTOCOL_VERSION: u32 = 3;
 
 #[tokio::main]
 async fn main() -> anyhow::Result<()> {
+    let args = match cli::parse(std::env::args().skip(1)) {
+        Ok(args) => args,
+        Err(msg) => {
+            eprintln!("uo-link-sidecar: {msg}\n\n{}", cli::USAGE);
+            std::process::exit(2);
+        }
+    };
+
+    match args.mode {
+        cli::Mode::Help => {
+            print!("{}", cli::USAGE);
+            return Ok(());
+        }
+        cli::Mode::Version => {
+            println!(
+                "uo-link-sidecar {} (protocol {})",
+                env!("CARGO_PKG_VERSION"),
+                PROTOCOL_VERSION
+            );
+            return Ok(());
+        }
+        // Tracing stays uninitialized here on purpose: the subscriber writes to stdout, and stdout
+        // is the document. Config::load's messages are dropped rather than interleaved into JSON
+        // an installer is about to parse — everything they would have said is in the document.
+        cli::Mode::PrintConfig => {
+            let loaded = config::Config::load(args.config.as_deref())?;
+            println!("{:#}", config::describe(&loaded));
+            return Ok(());
+        }
+        cli::Mode::Run => {}
+    }
+
     init_tracing();
     info!("uo-link sidecar starting");
 
-    let cfg = config::Config::load()?;
+    let loaded = config::Config::load(args.config.as_deref())?;
+    let cfg = loaded.cfg;
     info!(
+        config = %loaded.path.display(),
         shard = %cfg.shard.bind,
         web = %cfg.web.bind,
+        db = %cfg.store.path,
         auth = cfg.auth_required(),
         "configuration loaded"
     );