Queue SQLite writes on one connection so none fails as locked (closes #223)

internal/database now opens the database with one connection. pixa's
own reads and writes run on it one at a time instead of competing for
SQLite's lock, where a write that kept losing could wait past the
five-second busy timeout and fail with "database is locked". The busy
timeout stays, for another program writing to the same file.

With one connection, a query run while rows or a transaction are still
open would wait forever. No code in internal/database or
internal/imgcache does that; the comment on DB() tells callers.
README.md says pixa uses one connection and that requests wait while
eviction runs one of its queries.

Model: opus-5-5

internal/database/concurrent_writes_internal_test.go

@@ -13,11 +13,10 @@ import (
 )
 
 // TestConcurrentWritesAllSucceed opens a database the way pixad does and
-// writes to it from several goroutines at once, so the writes run on
-// separate connections, as one request's writes and the background eviction
-// pass do. Every write must succeed, none failing with "database is locked",
-// whether or not db_url already has parameters, and the parameters it has
-// must still apply.
+// writes to it from several goroutines at once, as one request's writes and
+// the background eviction pass do. Every write must succeed, none failing
+// with "database is locked", whether or not db_url already has parameters,
+// and the parameters it has must still apply.
 func TestConcurrentWritesAllSucceed(t *testing.T) {
 	t.Parallel()