Added the catalogue models and seed to support phase 5 of the POC prototype in the Persistence package.

Packages/Persistence/Sources/Public/Models/Series.swift

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+/// A `Sendable` snapshot of a series in the catalogue: a run of issues, which is what lets a gap in it show.
+public struct Series: Sendable, Equatable {
+
+    // MARK: Properties
+
+    /// The series' URL slug: its name, slugged.
+    public let slug: String
+    /// The series' name, e.g. `Nightward`.
+    public let name: String
+    /// The publisher's name.
+    public let publisher: String
+    /// The years the series ran, e.g. `2019–2021`.
+    public let years: String
+    /// The number of issues in the run; zero for a series the catalogue only knows from its listings.
+    public let issueCount: Int
+    /// The series in a sentence or two.
+    public let blurb: String
+
+    // MARK: Initializers
+
+    public init(
+        slug: String,
+        name: String,
+        publisher: String,
+        years: String,
+        issueCount: Int,
+        blurb: String
+    ) {
+        self.slug = slug
+        self.name = name
+        self.publisher = publisher
+        self.years = years
+        self.issueCount = issueCount
+        self.blurb = blurb
+    }
+
+}