fix: serialize HTTP headers as JSON so tool detection and bounty extraction work

templates/decnet_logging.py calls str(v) on all SD-PARAM values, turning a
headers dict into Python repr ('{'User-Agent': ...}') rather than JSON.
detect_tools_from_headers() called json.loads() on that string and silently
swallowed the error, returning [] for every HTTP event. Same bug prevented
the ingester from extracting User-Agent bounty fingerprints.

- templates/http/server.py: wrap headers dict in json.dumps() before passing
  to syslog_line so the value is a valid JSON string in the syslog record
- behavioral.py: add ast.literal_eval fallback for existing DB rows that were
  stored with the old Python repr format
- ingester.py: parse headers as JSON string in _extract_bounty so User-Agent
  fingerprints are stored correctly going forward
- tests: add test_json_string_headers and test_python_repr_headers_fallback
  to exercise both formats in detect_tools_from_headers

decnet/profiler/behavioral.py

@@ -314,12 +314,24 @@ def detect_tools_from_headers(events: list[LogEvent]) -> list[str]:
         if not raw_headers:
             continue
 
-        # headers may arrive as a JSON string or a dict already
+        # headers may arrive as a JSON string, a Python-repr string (legacy),
+        # or a dict already (in-memory / test paths).
         if isinstance(raw_headers, str):
             try:
                 headers: dict[str, str] = json.loads(raw_headers)
             except (json.JSONDecodeError, ValueError):
-                continue
+                # Backward-compat: events written before the JSON-encode fix
+                # were serialized as Python repr via str(dict).  ast.literal_eval
+                # handles that safely (no arbitrary code execution).
+                try:
+                    import ast as _ast
+                    _parsed = _ast.literal_eval(raw_headers)
+                    if isinstance(_parsed, dict):
+                        headers = _parsed
+                    else:
+                        continue
+                except Exception:
+                    continue
         elif isinstance(raw_headers, dict):
             headers = raw_headers
         else: