Build every database's schema from embedded SQL files (closes #56)

The main database, each webhook's event database and each archive file
get their schema from SQL files under internal/db/migrations/, one
directory per kind. Opening a file runs, in one transaction, each file
that schema_migrations does not list. GORM stays for queries.
AutoMigrate is gone, tests included.

An event database holds only its own tables now, so the sweep that
removed target rows copied into it is deleted. The association guard
stays: without it a delivery write fails on the missing targets table.

AutoMigrate also had GORM parse every model before a handle was shared.
That is now done on its own: GORM races when two goroutines first use
models that refer to each other.

Model: opus-5-5

internal/database/event_tier_indexes_test.go

@@ -13,10 +13,8 @@ import (
 	"sneak.berlin/go/webhooker/internal/database"
 )
 
-// TestWebhookDBManager_OpenAddsEventTierIndexes verifies that opening a
-// per-webhook database that predates these indexes creates them. It
-// stands in for an older database file by dropping the indexes
-// AutoMigrate just created, then reopening the same file.
+// TestWebhookDBManager_OpenAddsEventTierIndexes verifies that a new
+// per-webhook database has these indexes.
 func TestWebhookDBManager_OpenAddsEventTierIndexes(t *testing.T) {
 	t.Parallel()
 
@@ -42,27 +40,9 @@ func TestWebhookDBManager_OpenAddsEventTierIndexes(t *testing.T) {
 	db, err := mgr.GetDB(webhookID)
 	require.NoError(t, err)
 
-	// A fresh database has them.
-	for _, ix := range indexes {
-		require.True(t, db.Migrator().HasIndex(ix.model, ix.name))
-	}
-
-	// Stand in for a database file created before the indexes existed.
-	for _, ix := range indexes {
-		require.NoError(t, db.Migrator().DropIndex(ix.model, ix.name))
-		require.False(t, db.Migrator().HasIndex(ix.model, ix.name))
-	}
-
-	// Drop the cached connection so the next open reopens the file and
-	// runs AutoMigrate against it, as a restart would.
-	require.NoError(t, mgr.CloseAll())
-
-	db, err = mgr.GetDB(webhookID)
-	require.NoError(t, err)
-
 	for _, ix := range indexes {
 		assert.True(t, db.Migrator().HasIndex(ix.model, ix.name),
-			"opening the existing database should create %s", ix.name)
+			"a new database should have %s", ix.name)
 	}
 }