feat(backend): implement session coverage, resumable ingestion, deep year backfill, backoff rate-limit, and dynamic cache TTLs

internal/store/migrations/004_coverage.sql

@@ -0,0 +1,9 @@
+CREATE TABLE IF NOT EXISTS session_coverage (
+    session_key  INTEGER NOT NULL,
+    dataset      TEXT    NOT NULL,
+    status       TEXT    NOT NULL DEFAULT 'pending',
+    row_count    INTEGER NOT NULL DEFAULT 0,
+    updated_at   TEXT    NOT NULL DEFAULT (datetime('now')),
+    error_msg    TEXT,
+    PRIMARY KEY (session_key, dataset)
+);