refactor: address code review feedback for auth and pagination improvements

- Extract duplicated thread/message pagination functions to shared utils module
- Refactor pagination token parsing to use Option combinators instead of defaults
- Split access token generation from assignment for clearer error handling
- Add appservice token collision detection at startup and registration
- Allow appservice re-registration with same token (for config updates)
- Simplify thread relation chunk building using iterator chaining
- Fix saturating_inc edge case in relation queries with explicit filtering
- Add concise comments explaining non-obvious behaviour choices

src/api/client/utils.rs

@@ -0,0 +1,28 @@
+use conduwuit::{
+	Result, err,
+	matrix::pdu::{PduCount, ShortEventId},
+};
+
+/// Parse a pagination token, trying ShortEventId first, then falling back to
+/// PduCount
+pub(crate) fn parse_pagination_token(token: &str) -> Result<PduCount> {
+	// Try parsing as ShortEventId first
+	if let Ok(shorteventid) = token.parse::<ShortEventId>() {
+		// ShortEventId maps directly to a PduCount in our database
+		Ok(PduCount::Normal(shorteventid))
+	} else if let Ok(count) = token.parse::<u64>() {
+		// Fallback to PduCount for backwards compatibility
+		Ok(PduCount::Normal(count))
+	} else if let Ok(count) = token.parse::<i64>() {
+		// Also handle negative counts for backfilled events
+		Ok(PduCount::from_signed(count))
+	} else {
+		Err(err!(Request(InvalidParam("Invalid pagination token"))))
+	}
+}
+
+/// Convert a PduCount to a token string (using the underlying ShortEventId)
+pub(crate) fn count_to_token(count: PduCount) -> String {
+	// The PduCount's unsigned value IS the ShortEventId
+	count.into_unsigned().to_string()
+}