fix: Resolve Clippy CI failures from elided lifetime warnings

The latest Rust nightly compiler (2025-08-27) introduced the
elided-named-lifetimes lint which causes Clippy CI checks to fail
when an elided lifetime ('_) resolves to a named lifetime that's
already in scope.

This commit fixes the Clippy warnings by:
- Making lifetime relationships explicit where 'a is already in scope
- Keeping elided lifetimes ('_) in functions without explicit
  lifetime parameters
- Ensuring proper lifetime handling in the database pool module

Affected files (17 total):
- Database map modules: Handle, Key, and KeyVal references in get,
  qry, keys, and stream operations
- Database pool module: into_recv_seek function

This change resolves the CI build failures without changing any
functionality, ensuring the codebase remains compatible with the
latest nightly Clippy checks.

src/service/rooms/read_receipt/data.rs

@@ -65,7 +65,7 @@ impl Data {
 		&'a self,
 		room_id: &'a RoomId,
 		since: u64,
-	) -> impl Stream<Item = ReceiptItem<'_>> + Send + 'a {
+	) -> impl Stream<Item = ReceiptItem<'a>> + Send + 'a {
 		type Key<'a> = (&'a RoomId, u64, &'a UserId);
 		type KeyVal<'a> = (Key<'a>, CanonicalJsonObject);
 

src/service/rooms/read_receipt/mod.rs

@@ -112,7 +112,7 @@ impl Service {
 		&'a self,
 		room_id: &'a RoomId,
 		since: u64,
-	) -> impl Stream<Item = ReceiptItem<'_>> + Send + 'a {
+	) -> impl Stream<Item = ReceiptItem<'a>> + Send + 'a {
 		self.db.readreceipts_since(room_id, since)
 	}