feat(backend): chat context compaction

apps/backend/.env.example

@@ -49,4 +49,21 @@ PLATYPUS_SANDBOX_DOCKER_ENABLED=false
 # PLATYPUS_SANDBOX_DOCKER_ALLOWED_NETWORKS=shared-services,public-tools
 
 # Frontend URL for generating resource links in tool responses
-FRONTEND_URL=http://localhost:3001
+FRONTEND_URL=http://localhost:3001
+
+# Context compaction (ADR-0012 §Config & kill switch).
+# Compaction behavior is global; window/output size stays per-model.
+# COMPACTION_ENABLED=false disables proactive compaction (recovery still runs).
+# COMPACTION_ENABLED=true
+#
+# Optional overrides for the global ceiling. Unset = built-in defaults
+# (trigger 0.8, target 0.5, reserve 0.05, keepRecent 10, minPrunable 2000,
+#  minRecentPrunable 10000).
+# Lower the trigger to exercise auto-compaction on test deployments.
+# Keep target < trigger or compaction re-fires every turn.
+# COMPACTION_TRIGGER_RATIO=0.8
+# COMPACTION_TARGET_RATIO=0.5
+# COMPACTION_RESERVE_RATIO=0.05
+# COMPACTION_KEEP_RECENT=10
+# COMPACTION_MIN_PRUNABLE_CHARS=2000
+# COMPACTION_MIN_RECENT_PRUNABLE_CHARS=10000

apps/backend/drizzle/0046_context_compaction.sql

@@ -0,0 +1,5 @@
+ALTER TABLE "chat" ADD COLUMN "context_summary" text;--> statement-breakpoint
+ALTER TABLE "chat" ADD COLUMN "summary_watermark" text;--> statement-breakpoint
+ALTER TABLE "chat" ADD COLUMN "compaction_dirty" boolean DEFAULT false NOT NULL;--> statement-breakpoint
+ALTER TABLE "chat" ADD COLUMN "version" integer DEFAULT 0 NOT NULL;--> statement-breakpoint
+ALTER TABLE "provider" ADD COLUMN "model_meta" jsonb;