perf: migrate hot-path JSON serialization to orjson

stdlib json was FastAPI's default. Every response body, every SSE frame,
and every add_log/state/payload write paid the stdlib encode cost.

- pyproject.toml: add orjson>=3.10 as a core dep.
- decnet/web/api.py: default_response_class=ORJSONResponse on the
  FastAPI app, so every endpoint return goes through orjson without
  touching call sites. Explicit JSONResponse sites in the validation
  exception handlers migrated to ORJSONResponse for consistency.
- health endpoint's explicit JSONResponse → ORJSONResponse.
- SSE stream (api_stream_events.py): 6 json.dumps call sites →
  orjson.dumps(...).decode() — the per-event frames that fire on every
  sse tick.
- sqlmodel_repo.py: encode sites on the log-insert path switched to
  orjson (fields, payload, state value). Parser sites (json.loads)
  left as-is for now — not on the measured hot path.

decnet/web/db/sqlmodel_repo.py

@@ -13,6 +13,8 @@ from __future__ import annotations
 
 import asyncio
 import json
+
+import orjson
 import uuid
 from datetime import datetime, timezone
 from typing import Any, Optional, List
@@ -146,7 +148,7 @@ class SQLModelRepository(BaseRepository):
     async def add_log(self, log_data: dict[str, Any]) -> None:
         data = log_data.copy()
         if "fields" in data and isinstance(data["fields"], dict):
-            data["fields"] = json.dumps(data["fields"])
+            data["fields"] = orjson.dumps(data["fields"]).decode()
         if "timestamp" in data and isinstance(data["timestamp"], str):
             try:
                 data["timestamp"] = datetime.fromisoformat(
@@ -391,7 +393,7 @@ class SQLModelRepository(BaseRepository):
     async def add_bounty(self, bounty_data: dict[str, Any]) -> None:
         data = bounty_data.copy()
         if "payload" in data and isinstance(data["payload"], dict):
-            data["payload"] = json.dumps(data["payload"])
+            data["payload"] = orjson.dumps(data["payload"]).decode()
 
         async with self._session() as session:
             dup = await session.execute(
@@ -478,7 +480,7 @@ class SQLModelRepository(BaseRepository):
             result = await session.execute(statement)
             state = result.scalar_one_or_none()
 
-            value_json = json.dumps(value)
+            value_json = orjson.dumps(value).decode()
             if state:
                 state.value = value_json
                 session.add(state)