perf(tests): copy a migrated template DB instead of migrating per test

Most tests opened a throwaway panel DB with database.InitDB, which runs the
full AutoMigrate + seed on an empty file every time: ~230ms, and ~850ms under
-race because GORM's reflection-heavy migration is what the detector slows
most. internal/web/service does this in ~550 of its 830 tests, so the CI race
job spent ~10 of its ~14.6 minutes re-migrating empty databases.

internal/database/dbtest.InitDB migrates once per test process, then hands
each test its own copy of that file (~130ms under -race) and registers the
CloseDB cleanup. The copy then goes through InitDB like a panel restart, so
every test still starts from the state a fresh install has. Tests that reopen
an existing file, migrate a hand-built legacy DB or target Postgres keep
calling database.InitDB.

Locally under -race: internal/web/service 626s (last CI run) -> 114s,
internal/sub 246s -> 35s.

.github/claude/issue-analyst-context.md

@@ -100,8 +100,8 @@ question it already answers.
   subtests and `t.Helper()` on helpers. An assertion must pin the exact value,
   typed error or emitted string — `err != nil` and `len(x) > 0` are findings,
   not nits. Prefer real dependencies: a throwaway DB via
-  `database.InitDB(filepath.Join(t.TempDir(), "x-ui.db"))` with `t.Cleanup`, and
-  `httptest` for HTTP. `internal/sub`'s `initSubDB(t)` is the template.
+  `dbtest.InitDB(t, filepath.Join(t.TempDir(), "x-ui.db"))`
+  (`internal/database/dbtest`), and `httptest` for HTTP. `internal/sub`'s `initSubDB(t)` is the template.
   A test must FAIL without its fix; one that passes either way certifies
   nothing and then gets cited as proof the fix works.