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/schemas.py

@@ -66,6 +66,9 @@ class ScenarioBase(BaseModel):
     authors_note: Prose = ""
     ai_instructions: Prose = ""
     tags: Tags = ""
+    # Phase 12: RPG world-state template (stat defs, bands, rules, milestones).
+    # None means no RPG layer.
+    stat_schema: dict | None = None
 
 
 class ScenarioCreate(ScenarioBase):
@@ -80,6 +83,7 @@ class ScenarioUpdate(BaseModel):
     authors_note: Prose | None = None
     ai_instructions: Prose | None = None
     tags: Tags | None = None
+    stat_schema: dict | None = None
     script_ids: list[int] | None = None