feat(fusion_accounting_reports): MV for per-account-per-month balances

Made-with: Cursor

fusion_accounting_reports/data/sql/create_mv_account_balance.sql

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+-- Materialized view: per-account aggregated balances by year-month.
+-- Used by GL drill-down + trial balance for large DBs.
+-- Refresh strategy: cron every 15 minutes (Task 25); CONCURRENTLY-capable
+-- thanks to the unique index.
+
+CREATE MATERIALIZED VIEW IF NOT EXISTS fusion_account_balance_mv AS
+SELECT
+    ROW_NUMBER() OVER (
+        ORDER BY account_id, company_id, DATE_TRUNC('month', date)
+    )::INTEGER AS id,
+    account_id,
+    company_id,
+    DATE_TRUNC('month', date)::date AS period_month,
+    SUM(debit) AS debit,
+    SUM(credit) AS credit,
+    SUM(balance) AS balance,
+    COUNT(*) AS line_count
+FROM account_move_line
+WHERE parent_state = 'posted'
+GROUP BY account_id, company_id, DATE_TRUNC('month', date);
+
+-- The (account_id, company_id, period_month) tuple is the natural key.
+-- We mark it UNIQUE so REFRESH MATERIALIZED VIEW CONCURRENTLY is allowed.
+CREATE UNIQUE INDEX IF NOT EXISTS fusion_account_balance_mv_pkey
+    ON fusion_account_balance_mv (account_id, company_id, period_month);
+-- A separate index on the synthetic id is required by Odoo's ORM, which
+-- expects every model row to be addressable by `id`.
+CREATE UNIQUE INDEX IF NOT EXISTS fusion_account_balance_mv_id_idx
+    ON fusion_account_balance_mv (id);
+CREATE INDEX IF NOT EXISTS fusion_account_balance_mv_company_month
+    ON fusion_account_balance_mv (company_id, period_month);