Add --check command for configuration and connectivity validation

Adds `python -m app --check` which verifies:
- All required environment variables are set
- Matrix homeserver is reachable
- Bot access token is valid
- Bot is a member of the announcements room
- Database path is writable

Uses color-coded pass/fail output with actionable hints on failure.
Configures nio with a 10-second timeout and no retries so checks
complete quickly even against unreachable servers.

https://claude.ai/code/session_01DuzWyMMXvLMB4VxEwJyV4X

app/__main__.py

@@ -0,0 +1,35 @@
+"""CLI entry point for Melora.
+
+Usage:
+    python -m app --check    Verify configuration and connectivity
+    python -m app            Start the server
+"""
+
+import argparse
+import sys
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser(
+        prog="melora",
+        description="Melora — *arr media arrival webhook bridge for Matrix",
+    )
+    parser.add_argument(
+        "--check",
+        action="store_true",
+        help="verify configuration and connectivity, then exit",
+    )
+    args = parser.parse_args()
+
+    if args.check:
+        from app.check import run_checks
+
+        raise SystemExit(run_checks())
+
+    import uvicorn
+
+    uvicorn.run("app.main:app", host="0.0.0.0", port=8000)
+
+
+if __name__ == "__main__":
+    main()