Implement Phase 0+1: project setup, config, database, migrations

- Go module, Makefile, .golangci.yaml, .gitignore, example config
- TOML config loading with validation
- SQLite database with WAL, foreign keys, busy timeout
- Schema migrations: users, webauthn_credentials, notebooks, pages,
  strokes, share_links with indexes and cascading deletes
- 4 tests: open+migrate, idempotent, foreign keys, cascade delete

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

go.mod

@@ -0,0 +1,23 @@
+module git.wntrmute.dev/kyle/eng-pad-server
+
+go 1.25.0
+
+require (
+	github.com/pelletier/go-toml/v2 v2.3.0
+	github.com/spf13/cobra v1.10.2
+	modernc.org/sqlite v1.47.0
+)
+
+require (
+	github.com/dustin/go-humanize v1.0.1 // indirect
+	github.com/google/uuid v1.6.0 // indirect
+	github.com/inconshreveable/mousetrap v1.1.0 // indirect
+	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/ncruces/go-strftime v1.0.0 // indirect
+	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
+	github.com/spf13/pflag v1.0.9 // indirect
+	golang.org/x/sys v0.42.0 // indirect
+	modernc.org/libc v1.70.0 // indirect
+	modernc.org/mathutil v1.7.1 // indirect
+	modernc.org/memory v1.11.0 // indirect
+)