Implement the world-state summarization from the Phase 2 plan

Context was pure full-text replay of every message, capped at 40k
chars with oldest-dropped-first — no summarization/world-state layer,
as called out as still-missing in an earlier conversation.

Add a new update_world_state DM tool that persists a compact,
DM-authored recap (key NPCs, current location, open plot threads,
party/inventory state) to a new world_state table, one row per game.
It's injected into the system prompt every turn, ahead of the raw
message window. The DM is instructed to call it regularly — at every
scene change or major event, not just at the end — sending the full
current picture each time (matches the replace-not-merge pattern
already used for combat_stats/abilities/equipment).

Since the summary now backs up everything older, shrink the raw
message window from 40k to 16k chars — it's recent continuity now,
not the sole memory of the session. Full history remains available
via "Volltext laden" regardless, since that reads the messages table
directly rather than through this context builder.

Simplified from the original plan sketch (state JSONB) to a single
free-text summary field — natural-language recaps are something an
LLM authors well; a structured world model would need a schema the
DM would have to conform to for no real benefit here.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/llm/context.py

@@ -7,8 +7,10 @@ from app.models.character import Character
 from app.models.message import Message
 from app.models.user import User
 
-# Rough char-based budget for Phase 1 (~4 chars/token, leaves headroom for system prompt + output).
-MAX_CONTEXT_CHARS = 40_000
+# Rough char-based budget for the raw message window (~4 chars/token). Kept fairly small now that
+# update_world_state carries older context forward — this window is just recent continuity, not
+# the sole memory of the session anymore. Full history is still available via "Volltext laden".
+MAX_CONTEXT_CHARS = 16_000
 
 
 async def build_context(session: AsyncSession, game_id: uuid.UUID) -> list[dict]: