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/pdu_metadata/data.rs

@@ -60,7 +60,7 @@ impl Data {
 		target: ShortEventId,
 		from: PduCount,
 		dir: Direction,
-	) -> impl Stream<Item = (PduCount, impl Event)> + Send + '_ {
+	) -> impl Stream<Item = (PduCount, impl Event)> + Send + 'a {
 		// Query from exact position then filter excludes it (saturating_inc could skip
 		// events at min/max boundaries)
 		let from_unsigned = from.into_unsigned();