refactor(store): move the persistence layer into a shared FlipcashStore package

[bmc08gt]

Third of four PRs behind the push-preload work. Stacked on #754.

The problem

The notification service extension cannot reach Database while it lives in the app target. Xcode's synchronised file groups do not offer a way in: all three PBXFileSystemSynchronizedBuildFileExceptionSet entries in this project list Info.plist as an exclusion, which is the only thing the mechanism does. A file cannot join a second target that way, so sharing code with the extension means a package.

What moved

Flipcash/Core/Controllers/Database/ becomes FlipcashStore, a new target in FlipcashCore linked into both Flipcash and NotificationService. It is separate from FlipcashCore rather than part of it so that everything depending on the models does not also pull in SQLite. The SQLite.swift dependency is branch-pinned to match the app project's reference to the same fork, because SPM resolves one version of it for the whole graph.

NotificationContent is deliberately not wired to the package. It reads NotificationPreviewCache, the JSON side-car, and nothing in this PR changes that.

The move itself is mechanical: types the app target already used become public, and import FlipcashStore is added to the 28 app files and 38 test files that read through the database. No behaviour change.

Two things that did not move cleanly

• Updateable stayed in the app target. It is a SwiftUI @Observable wrapper that re-queries on .databaseDidChange, not persistence. The app target defaults new types to @MainActor and a package target does not, so in the package its init could not send self into a @MainActor task.
• Database is open rather than public, so the test bundle's TempDatabase can keep tying temp-file cleanup to the database's lifetime. Its members stay public: a subclass can add state but cannot override behaviour. The alternative was rewriting 96 makeTemp() call sites around a wrapper type.

Where the tests live

The database tests stay in the app test bundle rather than moving to a package test target, so they keep running on the simulator. The two failures this store hit during #754 — SQLITE_CANTOPEN on a read-only WAL open with no -shm, and SQLITE_IOERR from same-inode aliasing across the move — are iOS-specific and did not reproduce on macOS. A package test target would have hidden both.

Next

PR 4 makes the extension write its prefetched messages into the store instead of only into the side-car.