Phase 4 seam: in-memory DatabaseService + DB injection for ContextSelectionService

DatabaseService gains a testable init(dbQueue:) plus a makeInMemory()
convenience and schema-introspection helpers (tableExists/columnNames),
so migration tests don't need the test target to import GRDB directly.
ContextSelectionService now takes an injected DatabaseService (defaults
to .shared) and its DB-coupled methods (smartSelection,
getSummariesForExcludedRange, isMessageStarred) are bumped to internal.
No logic changes to existing call sites.

oAI/Services/DatabaseService.swift

@@ -154,7 +154,14 @@ final class DatabaseService: Sendable {
         (try? isoStyle.parse(string)) ?? (try? Date(string, strategy: .iso8601))
     }
 
-    nonisolated private init() {
+    /// Test seam: build a DatabaseService around a caller-provided queue (e.g. an in-memory `DatabaseQueue()`).
+    /// Each test should create its own fresh instance — never mutate `.shared` from a test.
+    nonisolated init(dbQueue: DatabaseQueue) {
+        self.dbQueue = dbQueue
+        try! migrator.migrate(dbQueue)
+    }
+
+    nonisolated private convenience init() {
         let fileManager = FileManager.default
         let appSupport = fileManager.urls(for: .applicationSupportDirectory, in: .userDomainMask).first!
         let dbDirectory = appSupport.appendingPathComponent("oAI", isDirectory: true)
@@ -163,12 +170,25 @@ final class DatabaseService: Sendable {
 
         let dbPath = dbDirectory.appendingPathComponent("oai_conversations.db").path
         Log.db.info("Opening database at \(dbPath)")
-        dbQueue = try! DatabaseQueue(path: dbPath)
-
-        try! migrator.migrate(dbQueue)
+        self.init(dbQueue: try! DatabaseQueue(path: dbPath))
     }
 
-    private nonisolated var migrator: DatabaseMigrator {
+    /// Test seam: a throwaway in-memory instance with all migrations applied. Avoids requiring
+    /// `import GRDB` from the test target just to construct a `DatabaseQueue()`.
+    nonisolated static func makeInMemory() -> DatabaseService {
+        DatabaseService(dbQueue: try! DatabaseQueue())
+    }
+
+    /// Test seam: schema introspection, so migration tests don't need `import GRDB` either.
+    nonisolated func tableExists(_ name: String) -> Bool {
+        (try? dbQueue.read { db in try db.tableExists(name) }) ?? false
+    }
+
+    nonisolated func columnNames(in table: String) -> [String] {
+        (try? dbQueue.read { db in try db.columns(in: table).map(\.name) }) ?? []
+    }
+
+    nonisolated var migrator: DatabaseMigrator {
         var migrator = DatabaseMigrator()
 
         migrator.registerMigration("v1") { db in