perf: use cached ParsedDecoded() instead of repeated json.Unmarshal (#1871)

## Problem

`StoreTx.ParsedDecoded()` already caches the result of `json.Unmarshal`
on first call via `sync.Once`. However, 13 call sites in `store.go` and
1 in `routes.go` were independently unmarshaling `DecodedJSON` into
local `map[string]interface{}` variables on every access, completely
ignoring the cache.

## Impact

For a store with 50k+ transmissions, each analytics endpoint that
iterates all packets re-parses 50k JSON strings per request. With
multiple endpoints, this means hundreds of thousands of redundant
`json.Unmarshal` calls per page load, each allocating new maps and
slices.

## Fix

Replace each `var d map[string]interface{};
json.Unmarshal([]byte(tx.DecodedJSON), &d)` with `d :=
tx.ParsedDecoded()`, which returns the cached parse result (parsed once,
reused forever).

### Call sites changed (14 total):
- `untrackAdvertPubkey` — advert PK extraction during eviction
- Ingestion path — payload field for API responses
- `evictStaleInternal` — node cleanup during eviction
- `GetAnalyticsTopology` — node PK extraction
- `GetAnalyticsHashCollisions` — advert PK extraction
- `GetAnalyticsDistance` — region node PK building
- `resolveAreaNodes` — node PK extraction
- `GetRecentPackets` — payload field for API response
- `routes.go` byType grouping — type field extraction

### Left unchanged (3 sites):
Three call sites that unmarshal into typed structs (`grpDec`,
`decodedMsg`, `decodedGrp`) cannot use `ParsedDecoded()` since they need
specific struct types.

## Testing
- `go build` passes
- No behavior change — same data, same logic, just avoids redundant
parses

---------

Co-authored-by: Joel Claw <358739783+Joel-Claw@users.noreply.github.com>

cmd/server/routes.go

@@ -2954,8 +2954,7 @@ func (s *Server) handleAudioLabBuckets(w http.ResponseWriter, r *http.Request) {
 			}
 			typeName := "UNKNOWN"
 			if tx.DecodedJSON != "" {
-				var d map[string]interface{}
-				if err := json.Unmarshal([]byte(tx.DecodedJSON), &d); err == nil {
+				if d := tx.ParsedDecoded(); d != nil {
 					if t, ok := d["type"].(string); ok && t != "" {
 						typeName = t
 					}

cmd/server/store.go

@@ -1798,8 +1798,8 @@ func (s *PacketStore) untrackAdvertPubkey(tx *StoreTx) {
 	if tx.PayloadType == nil || *tx.PayloadType != PayloadADVERT || tx.DecodedJSON == "" {
 		return
 	}
-	var d map[string]interface{}
-	if json.Unmarshal([]byte(tx.DecodedJSON), &d) != nil {
+	d := tx.ParsedDecoded()
+	if d == nil {
 		return
 	}
 	pk := ""
@@ -2916,9 +2916,14 @@ func (s *PacketStore) IngestNewFromDB(sinceID, limit int) ([]map[string]interfac
 			},
 		}
 		if tx.DecodedJSON != "" {
-			var payload map[string]interface{}
-			if json.Unmarshal([]byte(tx.DecodedJSON), &payload) == nil {
-				decoded["payload"] = payload
+			if payload := tx.ParsedDecoded(); payload != nil {
+				// Copy the cached map so broadcast consumers cannot mutate
+				// the shared cache or cause a data race (#1871 review).
+				cp := make(map[string]interface{}, len(payload))
+				for k, v := range payload {
+					cp[k] = v
+				}
+				decoded["payload"] = cp
 			}
 		}
 		// For TRACE packets, decode the full packet to include path.hopsCompleted
@@ -3189,9 +3194,14 @@ func (s *PacketStore) IngestNewObservations(sinceObsID, limit int) []map[string]
 			},
 		}
 		if tx.DecodedJSON != "" {
-			var payload map[string]interface{}
-			if json.Unmarshal([]byte(tx.DecodedJSON), &payload) == nil {
-				decoded["payload"] = payload
+			if payload := tx.ParsedDecoded(); payload != nil {
+				// Copy the cached map so broadcast consumers cannot mutate
+				// the shared cache or cause a data race (#1871 review).
+				cp := make(map[string]interface{}, len(payload))
+				for k, v := range payload {
+					cp[k] = v
+				}
+				decoded["payload"] = cp
 			}
 		}
 		// For TRACE packets, decode the full packet to include path.hopsCompleted
@@ -3737,8 +3747,8 @@ func (s *PacketStore) computeNodeHomeRegions() map[string]string {
 			continue
 		}
 
-		var d map[string]interface{}
-		if json.Unmarshal([]byte(tx.DecodedJSON), &d) != nil {
+		d := tx.ParsedDecoded()
+		if d == nil {
 			continue
 		}
 		pk, _ := d["pubKey"].(string)
@@ -4651,8 +4661,7 @@ func (s *PacketStore) evictStaleInternal(rpBatch map[int][]string) int {
 		// Must mirror indexByNode: process decoded JSON fields AND resolved_path pubkeys.
 		evictedFromNode := make(map[string]bool)
 		if tx.DecodedJSON != "" {
-			var decoded map[string]interface{}
-			if json.Unmarshal([]byte(tx.DecodedJSON), &decoded) == nil {
+			if decoded := tx.ParsedDecoded(); decoded != nil {
 				for _, field := range []string{"pubKey", "destPubKey", "srcPubKey"} {
 					if v, ok := decoded[field].(string); ok && v != "" {
 						if hashes, ok := s.nodeHashes[v]; ok {
@@ -5009,8 +5018,7 @@ func computeDistancesForTx(tx *StoreTx, nodeByPk map[string]*nodeInfo, repeaterS
 
 	var senderNode *nodeInfo
 	if tx.DecodedJSON != "" {
-		var dec map[string]interface{}
-		if json.Unmarshal([]byte(tx.DecodedJSON), &dec) == nil {
+		if dec := tx.ParsedDecoded(); dec != nil {
 			if pk, ok := dec["pubKey"].(string); ok && pk != "" {
 				senderNode = nodeByPk[pk]
 			}
@@ -8030,8 +8038,7 @@ func (s *PacketStore) computeAnalyticsHashSizes(region, area string) map[string]
 		var advertPK, advertName string
 		var advertParsed bool
 		if tx.PayloadType != nil && *tx.PayloadType == PayloadADVERT && tx.DecodedJSON != "" {
-			var d map[string]interface{}
-			if json.Unmarshal([]byte(tx.DecodedJSON), &d) == nil {
+			if d := tx.ParsedDecoded(); d != nil {
 				if v, ok := d["pubKey"].(string); ok {
 					advertPK = v
 				} else if v, ok := d["public_key"].(string); ok {
@@ -8342,8 +8349,7 @@ func (s *PacketStore) computeHashCollisions(region, area string) map[string]inte
 				}
 				// Collect node public keys from advert packets
 				if tx.DecodedJSON != "" {
-					var d map[string]interface{}
-					if json.Unmarshal([]byte(tx.DecodedJSON), &d) == nil {
+					if d := tx.ParsedDecoded(); d != nil {
 						if pk, ok := d["pubKey"].(string); ok && pk != "" {
 							regionNodePKs[pk] = true
 						}
@@ -8706,8 +8712,8 @@ func (s *PacketStore) computeNodeHashSizeInfo() map[string]*hashSizeNodeInfo {
 		}
 		hs := int((pathByte>>6)&0x3) + 1
 
-		var d map[string]interface{}
-		if json.Unmarshal([]byte(tx.DecodedJSON), &d) != nil {
+		d := tx.ParsedDecoded()
+		if d == nil {
 			continue
 		}
 		pk := ""
@@ -9645,8 +9651,8 @@ func (s *PacketStore) GetNodeAnalytics(pubkey string, days int) (*NodeAnalyticsR
 		if p.DecodedJSON == "" {
 			continue
 		}
-		var decoded map[string]interface{}
-		if json.Unmarshal([]byte(p.DecodedJSON), &decoded) != nil {
+		decoded := p.ParsedDecoded()
+		if decoded == nil {
 			continue
 		}
 		type candidate struct{ key, name string }