M2a: server workspace, kvitt-domain, skeleton routes, OpenAPI match, Dockerfile

Cargo workspace (kvitt-domain, kvitt-server, kvitt-import), Rust 1.99.1 pinned.
kvitt-domain passes every Rust vector and adds next_occurrence_date (D-015).
kvitt-server: config, SQLite pool (WAL, §8.9 pragmas), migration 0001 identical to
the contract schema, error type for all 25 codes, auth extractor and protocol
layer, all 33 routes (E-1/E-2 implemented, the rest 501), utoipa document equal
to contracts/openapi.yaml, seams for M3/M4/M9, serve/migrate/healthcheck.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/crates/kvitt-server/src/audit/mod.rs

@@ -0,0 +1,63 @@
+//! The audit log seam (SPEC §14.4.7). M2a defines the trait and a no-op; M4 owns `src/audit/**` and replaces the
+//! implementation with one that computes the field diff and inserts the `audit_log` row.
+//!
+//! Callers (sync push in M4, recurring generation in E-28 and the scheduler, the importer in M9) call
+//! [`AuditWriter::write`] inside their own `BEGIN IMMEDIATE` transaction, passing the wire object before and after
+//! the change; the writer derives `changes` per SPEC §14.4.7 and takes the entry's `server_version` from the same
+//! transaction.
+
+use std::future::Future;
+use std::pin::Pin;
+
+use sqlx::SqliteConnection;
+use uuid::Uuid;
+
+use crate::error::AppError;
+use crate::wire::{AuditAction, AuditSource, EntityType};
+
+/// A boxed, sendable future (object-safe async trait methods).
+pub type BoxFuture<'a, T> = Pin<Box<dyn Future<Output = T> + Send + 'a>>;
+
+/// One audit event to record.
+#[derive(Debug, Clone, PartialEq)]
+pub struct AuditEvent {
+    pub entity_type: EntityType,
+    pub entity_id: Uuid,
+    pub action: AuditAction,
+    pub source: AuditSource,
+    pub actor_user_id: Uuid,
+    /// `None` for the scheduler and the importer.
+    pub actor_device_id: Option<Uuid>,
+    /// The pushed `updated_at` (for `delete`, the pushed `deleted_at`); server now for scheduler and importer.
+    pub client_at: String,
+    /// Server now.
+    pub server_at: String,
+    /// The full wire object (`Expense` or `Settlement` as JSON) before the change; `None` for `create`.
+    pub before: Option<serde_json::Value>,
+    /// The full wire object after the change.
+    pub after: serde_json::Value,
+}
+
+/// Writes audit entries.
+pub trait AuditWriter: Send + Sync + 'static {
+    /// Records `event` on `conn`, which is inside the caller's write transaction. Returns the entry's id.
+    fn write<'a>(
+        &'a self,
+        conn: &'a mut SqliteConnection,
+        event: AuditEvent,
+    ) -> BoxFuture<'a, Result<Uuid, AppError>>;
+}
+
+/// Writes nothing. Used until M4 lands its writer.
+#[derive(Debug, Clone, Copy, Default)]
+pub struct NoopAuditWriter;
+
+impl AuditWriter for NoopAuditWriter {
+    fn write<'a>(
+        &'a self,
+        _conn: &'a mut SqliteConnection,
+        _event: AuditEvent,
+    ) -> BoxFuture<'a, Result<Uuid, AppError>> {
+        Box::pin(async { Ok(Uuid::now_v7()) })
+    }
+}