feat(realism): operator-tunable planner weights via realism_config

New realism_config table (uuid PK + unique key) + two repo methods
(get/set) backs an admin-only GET/PUT /api/v1/realism/config surface.

The planner now exposes apply_payload(payload) / current_payload() /
reset_to_defaults() and reads its weights through mutable module
globals; pick() resolves the live values each call. Validation
catches negative weights, zero totals, out-of-range canary_probability,
unknown content_class names, and silently drops cross-list entries
(canary class on the user list, etc).

The orchestrator worker calls _refresh_realism_config(repo) on
startup and every 5 ticks (~5min at 60s interval). Operator changes
land within one refresh window with no bus signal — the simpler path
for a knob whose latency tolerance is minutes.

decnet/web/db/models/__init__.py

@@ -78,6 +78,7 @@ from .orchestrator import (
     OrchestratorEventsResponse,
 )
 from .realism import (
+    RealismConfig,
     SyntheticFile,
     SyntheticFilesResponse,
 )
@@ -231,6 +232,7 @@ __all__ = [
     "OrchestratorEvent",
     "OrchestratorEventsResponse",
     # realism
+    "RealismConfig",
     "SyntheticFile",
     "SyntheticFilesResponse",
     # logs