fix: observation timestamps, leaked fields, perf path normalization

- #178: Use strftime ISO 8601 format instead of datetime() for observation
  timestamps in all SQL queries (v3 + v2 views). Add normalizeTimestamp()
  helper for non-v3 paths that may store space-separated timestamps.

- #179: Strip internal fields (decoded_json, direction, payload_type,
  raw_hex, route_type, score, created_at) from ObservationResp. Only
  expose id, transmission_id, observer_id, observer_name, snr, rssi,
  path_json, timestamp — matching Node.js parity.

- #180: Remove _parsedDecoded and _parsedPath from node detail
  recentAdverts response. These internal/computed fields were leaking
  to the API. Updated golden shapes.json accordingly.

- #181: Use mux route template (GetPathTemplate) for perf stats path
  normalization, converting {param} to :param for Node.js parity.
  Fallback to hex regex for unmatched routes. Compile regexes once at
  package level instead of per-request.

fixes #178, fixes #179, fixes #180, fixes #181

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

cmd/server/coverage_test.go

@@ -47,7 +47,7 @@ func setupTestDBv2(t *testing.T) *DB {
 		);
 		CREATE VIEW packets_v AS
 			SELECT o.id, t.raw_hex,
-				datetime(o.timestamp, 'unixepoch') AS timestamp,
+				strftime('%Y-%m-%dT%H:%M:%fZ', o.timestamp, 'unixepoch') AS timestamp,
 				o.observer_id, o.observer_name,
 				o.direction, o.snr, o.rssi, o.score, t.hash, t.route_type,
 				t.payload_type, t.payload_version, o.path_json, t.decoded_json, t.created_at