feat: collapse repeated values in stream history

internal/migrations/056-duplicate-prune-schema.sql

@@ -0,0 +1,36 @@
+/*
+ * DUPLICATE PRUNE SCHEMA MIGRATION
+ *
+ * A stream that publishes the same value every day stores one row a day forever.
+ * Reads carry the last observation forward, so every row after the first in such
+ * a run answers exactly what the row before it already answered. This migration
+ * creates the configuration the pruner in 057 reads; the rule itself lives there.
+ *
+ * Nothing is deleted until an operator turns `enabled` on, which ships false.
+ */
+
+CREATE TABLE IF NOT EXISTS duplicate_prune_config (
+    id INT8 PRIMARY KEY DEFAULT 1 CHECK (id = 1),
+    -- Read by the scheduler, not by the actions. The actions are leader-gated and
+    -- do what they are told, so an operator can still drain by hand through a
+    -- signed exec-sql while the scheduled sweep stays off.
+    enabled BOOL NOT NULL DEFAULT false,
+    -- A record is only ever a candidate once it is this far in the past. 30 days
+    -- covers 98.8% of the duplicates on mainnet; a tighter window buys about 1%.
+    retention_days INT NOT NULL DEFAULT 30,
+    prune_schedule TEXT NOT NULL DEFAULT '0 */6 * * *',
+    -- Where the sweep is. Duplicate-ness is a property of a whole stream rather
+    -- than of one day, so there is no queue to drain the way digest has: the
+    -- pruner walks streams.id in order and wraps at the end. A stream with
+    -- nothing to prune costs one bounded query.
+    last_stream_ref INT NOT NULL DEFAULT 0,
+    updated_at_height INT8 NOT NULL DEFAULT 0,
+
+    CONSTRAINT chk_dpc_retention_days_positive CHECK (retention_days > 0),
+    CONSTRAINT chk_dpc_cursor_not_negative CHECK (last_stream_ref >= 0)
+);
+
+-- Seeded here on purpose. digest_config is not, and the consequence is a network
+-- where digest has simply never run because nobody noticed the row was missing.
+-- Creating it disabled costs nothing and removes that failure mode.
+INSERT INTO duplicate_prune_config (id) VALUES (1) ON CONFLICT (id) DO NOTHING;