Consume NIP-19 relay hints for event fetching

Extract and use relay hints from bech32 entities (nevent, nprofile, naddr)
and event tag references (e, q tags) to fetch events from hinted relays
not in the user's relay pool.

Changes:
- Parse relay hints from bech32 TLV data in URLHandler
- Pass relay hints through SearchType and NoteReference enums
- Add ensureConnected() to RelayPool for ephemeral relay connections
- Implement ephemeral relay lease management with race condition protection
- Add repostTarget() helper to extract relay hints from repost e tags
- Add QuoteRef struct to preserve relay hints from q tags (NIP-10/NIP-18)
- Support relay hints in replies with author pubkey in e-tags (NIP-10)
- Implement fallback broadcast when hinted relays don't respond
- Add comprehensive test coverage for relay hint functionality
- Add DEBUG logging for relay hint tracing during development

Implementation details:
- Connect to hinted relays as ephemeral, returning early when first connects
- Use total deadline to prevent timeout accumulation across hint attempts
- Decrement lease count before suspension points to ensure atomicity
- Fall back to broadcast if hints don't resolve or respond

Closes: https://github.com/damus-io/damus/issues/1147
Changelog-Added: Added relay hint support for nevent, nprofile, naddr links and event tag references (reposts, quotes, replies)
Signed-off-by: alltheseas
Signed-off-by: Daniel D'Aquino <daniel@daquino.me>
Co-authored-by: alltheseas <alltheseas@users.noreply.github.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: Daniel D'Aquino <daniel@daquino.me

nostrdb/NdbTagIterator.swift

@@ -42,6 +42,36 @@ struct TagSequence: Sequence {
     }
 }
 
+// MARK: - Relay Hint Extraction
+
+extension TagSequence {
+    /// Extracts a relay URL hint from position 2 of the tag, if present and valid.
+    ///
+    /// Per NIP-01 and NIP-10, position 2 in `e`, `p`, `a`, and `q` tags contains an optional
+    /// relay URL where the referenced entity may be found.
+    ///
+    /// Example tag: `["e", "<event-id>", "wss://relay.example.com"]`
+    ///
+    /// - Returns: A valid `RelayURL` if position 2 contains a non-empty, valid relay URL; `nil` otherwise.
+    var relayHint: RelayURL? {
+        guard count >= 3 else { return nil }
+        let urlString = self[2].string()
+        guard !urlString.isEmpty else { return nil }
+        return RelayURL(urlString)
+    }
+
+    /// Extracts relay hints from the tag as an array.
+    ///
+    /// Currently tags only support a single relay hint at position 2, but this method
+    /// returns an array for consistency with `NEvent.relays` and future extensibility.
+    ///
+    /// - Returns: An array containing the relay hint if present, or an empty array.
+    var relayHints: [RelayURL] {
+        guard let hint = relayHint else { return [] }
+        return [hint]
+    }
+}
+
 struct TagIterator: IteratorProtocol {
     typealias Element = NdbTagElem