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/db/migrations/archive/001_schema.sql

@@ -0,0 +1,18 @@
+-- Migration 001: the schema of a database target's archive file. The
+-- archivedEvent model in internal/delivery must agree with it.
+
+CREATE TABLE archived_events (
+    id integer PRIMARY KEY AUTOINCREMENT,
+    event_id text,
+    webhook_id text,
+    entrypoint_id text,
+    method text,
+    raw_query text,
+    headers text,
+    body text,
+    content_type text,
+    archived_at datetime
+);
+
+CREATE INDEX idx_archived_events_event_id ON archived_events (event_id);
+CREATE INDEX idx_archived_events_archived_at ON archived_events (archived_at);