docs: importance of receipt's SERIAL id in DB

Signed-off-by: Alexis Asseman <alexis@semiotic.ai>

Author: aasseman

migrations/20230912220523_tap_receipts.up.sql

@@ -1,5 +1,5 @@
 CREATE TABLE IF NOT EXISTS scalar_tap_receipts (
-    id BIGSERIAL PRIMARY KEY,
+    id BIGSERIAL PRIMARY KEY, -- id being SERIAL is important for the function of tap-agent
     allocation_id CHAR(40) NOT NULL,
     sender_address CHAR(40) NOT NULL,
     timestamp_ns NUMERIC(20) NOT NULL,

tap-agent/src/tap/sender_allocation_relationship.rs

@@ -43,6 +43,9 @@ type TapManager = tap_core::tap_manager::Manager<
 #[derive(Default, Debug)]
 struct UnaggregatedFees {
     pub value: u128,
+    /// The ID of the last receipt value added to the unaggregated fees value.
+    /// This is used to make sure we don't process the same receipt twice. Relies on the fact that
+    /// the receipts IDs are SERIAL in the database.
     pub last_id: u64,
 }