feat(spend_logs): opt-in native Postgres partitioning for SpendLogs retention (#29466)

High-volume deployments see LiteLLM_SpendLogs grow unbounded because
retention via DELETE leaves dead tuples that autovacuum cannot reclaim
fast enough. With a range-partitioned table, retention drops whole
partitions instead: an instant metadata operation that returns disk to
the OS immediately.

The feature is gated behind general_settings.use_spend_logs_partitioning
(default false). With the flag off, the cleanup job never queries the
catalog and behaves exactly as today. With it on, the job verifies the
table is partitioned, pre-creates upcoming partitions, and drops expired
ones; expired rows the drops cannot reach (DEFAULT partition, partitions
spanning the cutoff) are still deleted row-wise so retention is never
bypassed. If the table is not partitioned it falls back to batched
DELETE only.

Converting an existing table is a manual, documented operation in
db_scripts/partition_spend_logs.sql; db_scripts/unpartition_spend_logs.sql
rolls it back. Both scripts rename the old table's indexes aside before
recreating them, since a table rename keeps the schema-unique index names
and would otherwise silently skip the CREATE INDEX IF NOT EXISTS block.

Granularity and pre-create lookahead are tunable via
SPEND_LOG_PARTITION_INTERVAL (day/week/month, invalid values fall back to
day) and SPEND_LOG_PARTITION_PRECREATE_AHEAD.

db_scripts/unpartition_spend_logs.sql

@@ -0,0 +1,69 @@
+-- Rolls back db_scripts/partition_spend_logs.sql: converts the native
+-- range-partitioned "LiteLLM_SpendLogs" table back into a plain,
+-- non-partitioned table matching the default LiteLLM schema.
+--
+-- When/why: run this if you want to stop using partition-based retention and
+-- return to DELETE-based cleanup, or to restore the original single-column
+-- primary key ("request_id") that the partitioned layout had to widen to a
+-- composite ("request_id", "startTime").
+--
+-- IMPORTANT
+--   * Test on a staging copy first and take a backup.
+--   * Postgres cannot convert a partitioned table back in place, so this
+--     renames the partitioned table aside and creates a fresh plain table.
+--   * The composite PK could in principle hold the same "request_id" in more
+--     than one partition, so rows are copied with ON CONFLICT DO NOTHING to
+--     restore the single-column PK without failing on such duplicates.
+--   * For large tables the INSERT ... SELECT copies every surviving row and may
+--     run long; do it during a low-traffic window.
+--   * Also remove use_spend_logs_partitioning from proxy_config.yaml (or set it
+--     to false) so the cleanup job returns to DELETE-based retention.
+
+BEGIN;
+
+ALTER TABLE "LiteLLM_SpendLogs" RENAME TO "LiteLLM_SpendLogs_partitioned";
+
+-- Renaming a table does NOT rename its indexes, and index names are unique per
+-- schema. Move the partitioned table's indexes aside so the CREATE INDEX
+-- statements below actually create indexes on the new plain table instead of
+-- being silently skipped by IF NOT EXISTS, and so the new PK keeps the
+-- canonical name.
+ALTER INDEX IF EXISTS "LiteLLM_SpendLogs_pkey"
+    RENAME TO "LiteLLM_SpendLogs_partitioned_pkey";
+ALTER INDEX IF EXISTS "LiteLLM_SpendLogs_pkey1"
+    RENAME TO "LiteLLM_SpendLogs_partitioned_pkey1";
+ALTER INDEX IF EXISTS "LiteLLM_SpendLogs_startTime_idx"
+    RENAME TO "LiteLLM_SpendLogs_partitioned_startTime_idx";
+ALTER INDEX IF EXISTS "LiteLLM_SpendLogs_startTime_request_id_idx"
+    RENAME TO "LiteLLM_SpendLogs_partitioned_startTime_request_id_idx";
+ALTER INDEX IF EXISTS "LiteLLM_SpendLogs_end_user_idx"
+    RENAME TO "LiteLLM_SpendLogs_partitioned_end_user_idx";
+ALTER INDEX IF EXISTS "LiteLLM_SpendLogs_session_id_idx"
+    RENAME TO "LiteLLM_SpendLogs_partitioned_session_id_idx";
+
+CREATE TABLE "LiteLLM_SpendLogs" (
+    LIKE "LiteLLM_SpendLogs_partitioned" INCLUDING DEFAULTS INCLUDING GENERATED
+);
+
+ALTER TABLE "LiteLLM_SpendLogs"
+    ADD PRIMARY KEY ("request_id");
+
+CREATE INDEX IF NOT EXISTS "LiteLLM_SpendLogs_startTime_idx"
+    ON "LiteLLM_SpendLogs" ("startTime");
+
+CREATE INDEX IF NOT EXISTS "LiteLLM_SpendLogs_startTime_request_id_idx"
+    ON "LiteLLM_SpendLogs" ("startTime", "request_id");
+
+CREATE INDEX IF NOT EXISTS "LiteLLM_SpendLogs_end_user_idx"
+    ON "LiteLLM_SpendLogs" ("end_user");
+
+CREATE INDEX IF NOT EXISTS "LiteLLM_SpendLogs_session_id_idx"
+    ON "LiteLLM_SpendLogs" ("session_id");
+
+INSERT INTO "LiteLLM_SpendLogs"
+SELECT * FROM "LiteLLM_SpendLogs_partitioned"
+ON CONFLICT ("request_id") DO NOTHING;
+
+DROP TABLE "LiteLLM_SpendLogs_partitioned";
+
+COMMIT;