Import existing LinTunes project

Snapshot of the existing codebase before working through the TASKS.md
backlog. Real library data (data/) and the iTunes import fixture
(itunes-test-library/) are gitignored.

lintunes/inhibit.py

@@ -0,0 +1,45 @@
+"""Keep the machine awake while music is playing.
+
+Uses the GNOME SessionManager D-Bus interface (the same session bus PyQt6's
+QtDBus already talks to for MPRIS) to register a *suspend* inhibitor. GNOME's
+critical-battery action overrides this inhibitor, so a near-dead laptop still
+suspends as expected — we only block idle/automatic suspend while playing.
+
+On non-GNOME desktops the interface won't be available; every method then
+no-ops silently rather than failing.
+"""
+from PyQt6.QtDBus import QDBusConnection, QDBusInterface, QDBusReply
+
+
+SERVICE = "org.gnome.SessionManager"
+PATH = "/org/gnome/SessionManager"
+INHIBIT_SUSPEND = 4  # GsmInhibitorFlag: inhibit suspending the session/computer
+APP_ID = "org.lintunes.LinTunes"
+REASON = "Playing music"
+
+
+class SleepInhibitor:
+    def __init__(self):
+        self._cookie: int | None = None
+        bus = QDBusConnection.sessionBus()
+        self._iface = QDBusInterface(SERVICE, PATH, SERVICE, bus)
+
+    def _available(self) -> bool:
+        return self._iface.isValid()
+
+    def inhibit(self):
+        """Block automatic suspend. No-op if already inhibiting or unavailable."""
+        if self._cookie is not None or not self._available():
+            return
+        reply = QDBusReply(self._iface.call(
+            "Inhibit", APP_ID, 0, REASON, INHIBIT_SUSPEND))
+        if reply.isValid():
+            self._cookie = int(reply.value())
+
+    def release(self):
+        """Release a held inhibitor, if any."""
+        if self._cookie is None:
+            return
+        if self._available():
+            self._iface.call("Uninhibit", self._cookie)
+        self._cookie = None