Prune LangGraph checkpoints per thread and delete them with the conversation

Every graph step stores a full snapshot of the conversation, nothing pruned
old ones, and deleting a conversation left its thread behind (about 1.6 MB per
checkpoint at 200 turns). After each turn, only the newest
CHECKPOINT_KEEP_PER_THREAD (default 20) checkpoints of that thread are kept,
plus their pending writes; state reads only use the newest, and pruning a
200-checkpoint thread takes about 2 ms. Deleting a conversation also deletes
its thread, without failing the request if the checkpoint store errors.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.env.example

@@ -204,6 +204,9 @@ OBSIDIAN_VAULT_NAME=
 # LangGraph checkpoint store (SQLite). Must be on a persistent volume.
 # Both blue and green containers share this volume via docker-compose.
 CHECKPOINT_DB_PATH=/app/checkpoints/checkpoints.db
+# Checkpoints keep a full message-list snapshot per turn; this bounds how many
+# are kept per conversation thread (older ones are pruned after each turn).
+CHECKPOINT_KEEP_PER_THREAD=20
 
 # Beli restaurant recommendations
 # Beli account credentials for each profile are stored encrypted in the database.