store: give ecosystem traces their own table

Traces were written as facts. A single Praxis action wrote several of
them, so machine-rate rows crowded out the bounded fact readers that
humans and evaluation consume. The habit profile window of 2000 facts
and the memeval snapshot both filled with call records instead of what
Maven learned about the owner. Traces now go to ecosystem_traces, with
correlation, causation, duration and HTTP status as columns, pruned to
the most recent 5000. The new reader is exposed over IPC and rendered
as the Calls card on the ecosystem page, so it is a table someone
actually looks at.

Found in review of #84.

internal/store/migrations.go

@@ -160,6 +160,30 @@ ALTER TABLE reminders ADD COLUMN next_fire_ts INTEGER;`, // #2
 	);
 	CREATE UNIQUE INDEX IF NOT EXISTS idx_tasks_live_norm ON tasks (norm) WHERE status IN ('candidate','open');
 	CREATE INDEX IF NOT EXISTS idx_tasks_status ON tasks (status, created_ts DESC);`,
+
+	// #15 — ecosystem call traces (Vikunja #273). Deliberately NOT facts.
+	// Traces are written at machine rate, one act turn produces three or four,
+	// while facts are written at human rate. Sharing the facts table made every
+	// bounded reader of facts (the habit profile's 2000-row window, memeval's
+	// prompt snapshot, /dash's 50 and /history's 200) read mostly traces after
+	// a day of ecosystem use, pushing the rows that matter out of range.
+	// Retention is enforced on write (PruneEcosystemTraces) because nothing
+	// here is an audit trail: a trace answers "did this hop work" for as long
+	// as anyone is still asking.
+	`CREATE TABLE IF NOT EXISTS ecosystem_traces (
+		id             INTEGER PRIMARY KEY AUTOINCREMENT,
+		ts             INTEGER NOT NULL,
+		service        TEXT    NOT NULL,
+		operation      TEXT    NOT NULL,
+		status         TEXT    NOT NULL,
+		duration_ms    INTEGER NOT NULL DEFAULT 0,
+		correlation_id TEXT    NOT NULL DEFAULT '',
+		causation_id   TEXT    NOT NULL DEFAULT '',
+		http_status    INTEGER NOT NULL DEFAULT 0,
+		fields         TEXT    NOT NULL DEFAULT '{}'
+	);
+	CREATE INDEX IF NOT EXISTS idx_eco_traces_ts ON ecosystem_traces (ts DESC);
+	CREATE INDEX IF NOT EXISTS idx_eco_traces_correlation ON ecosystem_traces (correlation_id);`,
 }
 
 // migrate applies every migration with a number greater than the DB's current