Add Phase 12: native RPG world state

Structured world/player/NPC stats, two-way flags, and sticky milestones
per scenario (stat_schema). The AI proposes a per-turn delta; a Python
engine referees it (clamp to min/max, per-turn cap, cooldown, counters).
Band word-labels plus a fixed stat guide (descriptions + full ranges)
keep the model grounded. World State drawer + Insights delta report;
undo/retry roll it back via the Phase 11 snapshot pattern.

- migrations 26-28 (scenarios.stat_schema, adventures.world_state,
  actions.world_state_before); all nullable, additive, safe on existing rows
- migration 29 raises the default context budget 4096 -> 16384
  (custom values preserved)
- seeded demo scenario 04-rpg-world-state.json (Bandit Camp)
- 19 new tests (33 total pass)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/routers/scenarios.py

@@ -109,6 +109,7 @@ def export_scenario(
         "authorsNote": s.authors_note,
         "aiInstructions": s.ai_instructions,
         "tags": s.tags,
+        "statSchema": s.stat_schema,
         "storyCards": [
             {"type": c.type, "name": c.name, "keys": c.keys, "entry": c.entry, "notes": c.notes}
             for c in s.story_cards
@@ -137,6 +138,7 @@ _SCENARIO_KEYS = {
     "instructions": "ai_instructions",
 }
 _IGNORED_KEYS = {"format", "storyCards", "worldInfo", "worldInformation", "scripts", "tags",
+                 "statSchema", "stat_schema",
                  "createdAt", "updatedAt", "id", "publicId", "image", "nsfw", "type", "options"}
 
 
@@ -165,6 +167,10 @@ def import_scenario(
     elif isinstance(tags, str):
         fields["tags"] = tags
 
+    schema = bundle.get("statSchema") or bundle.get("stat_schema")
+    if isinstance(schema, dict):
+        fields["stat_schema"] = schema
+
     scenario = models.Scenario(**fields, user_id=user.id)
     if not scenario.title:
         scenario.title = "Imported Scenario"