Add --db and let the user choose where the database lives

- `kareer --db <path>` works for the GUI and every subcommand, before
  or after the subcommand name, and creates the file if missing. main()
  strips it from argv before CLI/GUI routing; every parser declares it
  so it shows in --help.
- Path precedence: --db > KAREER_DB_PATH > the location chosen in the
  GUI (kareerrc [Database] Path, read at startup for GUI and CLI alike)
  > $XDG_DATA_HOME/kareer/kareer.sqlite.
- First GUI run (nothing forced, no file yet, or the configured file
  has gone missing): DatabaseSetupDialog offers the default location, a
  folder of the user's choice, or an existing database used in place.
  Until then JobsDatabase opens nothing.
- Preferences gains a Database section: Move to... (copies, leaves the
  original), Open Existing..., Use Default Location. Refuses foreign
  SQLite files and unwritable ones.
- Models call JobsDatabase::reopenIfPathChanged() on refresh, so
  switching databases needs no restart.
- Link KF6::ConfigCore; add a QuickDialogs2 configure-time guard; add
  kconfig to the Arch dependencies and a note to the Flatpak manifest.
- README documents --db, the precedence, and the first-run choice, and
  gains Fedora 44 build dependencies; CONTRIBUTING gets a dev recipe.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01BxDf7HqD1xPnsZP8wt3NTk

src/jobsdatabase.h

@@ -31,10 +31,42 @@ public:
     JobsDatabase(const JobsDatabase &) = delete;
     JobsDatabase &operator=(const JobsDatabase &) = delete;
 
-    /// Default location: $XDG_DATA_HOME/kareer/kareer.sqlite, overridable
-    /// with the KAREER_DB_PATH environment variable (used by autotests).
+    /// The database file a default-constructed instance opens. Resolved in
+    /// order: the --db override, the KAREER_DB_PATH environment variable
+    /// (used by autotests), the path configured in kareerrc, and finally
+    /// $XDG_DATA_HOME/kareer/kareer.sqlite.
     static QString defaultPath();
 
+    /// $XDG_DATA_HOME/kareer/kareer.sqlite, ignoring every override.
+    static QString standardPath();
+
+    /// Set from the --db command-line option; wins over everything else.
+    static void setPathOverride(const QString &path);
+
+    /// The user's chosen location (kareerrc); empty means standardPath().
+    static void setConfiguredPath(const QString &path);
+    static QString configuredPath();
+
+    /// True when --db or KAREER_DB_PATH decides the path, so the configured
+    /// location has no effect.
+    static bool hasForcedPath();
+
+    /// While true (GUI first run, before the user has picked a location), a
+    /// default-constructed instance opens nothing and reports an error.
+    static void setSelectionPending(bool pending);
+    static bool selectionPending();
+
+    /// The file this instance opened, or empty if it opened nothing.
+    QString path() const;
+
+    /// Reopens against defaultPath() if that no longer matches path() (the
+    /// user picked another location). Returns true if it reopened.
+    bool reopenIfPathChanged();
+
+    /// Performs a harmless write (rewrites the header's user_version), so
+    /// callers can tell a read-only file apart from a usable one.
+    bool checkWritable();
+
     bool isOpen() const;
     QString lastError() const;
 
@@ -59,9 +91,11 @@ public:
 
 private:
     void init(const QString &path);
+    void close();
     bool migrate();
     Job jobFromQuery(class QSqlQuery &query) const;
 
     QString m_connectionName;
+    QString m_path;
     QString m_lastError;
 };