chore(db): use one SQLite database per folder (#10042)

This changes the database structure to use one database per folder, with
a small main database to coordinate. Reverts the prior change to buffer
all files in memory when pulling, meaning there is now a phase where the
WAL file will grow significantly, at least for initial sync of folders
with many directories.

---------

Co-authored-by: bt90 <btom1990@googlemail.com>

internal/db/sqlite/sql/schema/folder/40-counts.sql

@@ -0,0 +1,47 @@
+-- Copyright (C) 2025 The Syncthing Authors.
+--
+-- This Source Code Form is subject to the terms of the Mozilla Public
+-- License, v. 2.0. If a copy of the MPL was not distributed with this file,
+-- You can obtain one at https://mozilla.org/MPL/2.0/.
+
+-- Counts
+--
+-- Counts and sizes are maintained for each device, folder, type, flag bits
+-- combination.
+CREATE TABLE IF NOT EXISTS counts (
+    device_idx INTEGER NOT NULL,
+    type INTEGER NOT NULL,
+    local_flags INTEGER NOT NULL,
+    count INTEGER NOT NULL,
+    size INTEGER NOT NULL,
+    deleted INTEGER NOT NULL, -- boolean
+    PRIMARY KEY(device_idx, type, local_flags, deleted),
+    FOREIGN KEY(device_idx) REFERENCES devices(idx) ON DELETE CASCADE
+) STRICT, WITHOUT ROWID
+;
+
+--- Maintain counts when files are added and removed using triggers
+
+CREATE TRIGGER IF NOT EXISTS counts_insert AFTER INSERT ON files
+BEGIN
+    INSERT INTO counts (device_idx, type, local_flags, count, size, deleted)
+        VALUES (NEW.device_idx, NEW.type, NEW.local_flags, 1, NEW.size, NEW.deleted)
+        ON CONFLICT DO UPDATE SET count = count + 1, size = size + NEW.size;
+END
+;
+CREATE TRIGGER IF NOT EXISTS counts_delete AFTER DELETE ON files
+BEGIN
+    UPDATE counts SET count = count - 1, size = size - OLD.size
+        WHERE device_idx = OLD.device_idx AND type = OLD.type AND local_flags = OLD.local_flags AND deleted = OLD.deleted;
+END
+;
+CREATE TRIGGER IF NOT EXISTS counts_update AFTER UPDATE OF local_flags ON files
+WHEN NEW.local_flags != OLD.local_flags
+BEGIN
+    INSERT INTO counts (device_idx, type, local_flags, count, size, deleted)
+        VALUES (NEW.device_idx, NEW.type, NEW.local_flags, 1, NEW.size, NEW.deleted)
+        ON CONFLICT DO UPDATE SET count = count + 1, size = size + NEW.size;
+    UPDATE counts SET count = count - 1, size = size - OLD.size
+        WHERE device_idx = OLD.device_idx AND type = OLD.type AND local_flags = OLD.local_flags AND deleted = OLD.deleted;
+END
+;