refactor(db): gate postgres operations by backend

Signed-off-by: npub1z3hmzc9ryehxzedl5wzlvpyvja0d483peaja5zt6pd0209f9x2jspe2dxh <146fb160a3266e6165bfa385f6048c975eda9e21cf65da097a0b5ea7952532a5@buzz.block.builderlab.xyz>

crates/buzz-db/src/error.rs

@@ -13,6 +13,10 @@ pub enum DbError {
     #[error("migration error: {0}")]
     Migrate(#[from] sqlx::migrate::MigrateError),
 
+    /// Operation is unsupported by the selected database backend.
+    #[error("unsupported database backend operation: {0}")]
+    UnsupportedBackend(&'static str),
+
     /// Attempted to store an AUTH event (kind 22242), which is forbidden.
     #[error("AUTH events (kind 22242) must not be stored")]
     AuthEventRejected,