Add user model and SQLite storage

internal/storage/ provides:
- Open(path) to create or open the SQLite database with WAL journal,
  busy timeout, and foreign keys enabled
- Embedded migrations that create the users table on first run
- UserStore with Create, GetByID, GetByEmail, UpdateLastLogin, Delete
- Email normalisation (trim+lowercase) and uniqueness enforcement
  with ErrEmailTaken
- ErrNotFound on lookups and deletes
- UUIDv4 IDs auto-generated when caller leaves ID empty

Uses modernc.org/sqlite (pure-Go) so the binary stays CGO-free and
matches Dockerfile.dev's CGO_ENABLED=0.

Tests cover all CRUD operations, email uniqueness (case-insensitive),
WAL mode verification, and ErrNotFound paths.

Closes #8.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

go.mod

@@ -1,18 +1,29 @@
 module git.librete.ch/public/librenotes
 
-go 1.20
+go 1.25.0
+
+require (
+	github.com/google/uuid v1.6.0
+	github.com/junegunn/fzf v0.58.0
+	modernc.org/sqlite v1.50.0
+)
 
 require (
 	github.com/charlievieth/fastwalk v1.0.9 // indirect
+	github.com/dustin/go-humanize v1.0.1 // indirect
 	github.com/gdamore/encoding v1.0.1 // indirect
 	github.com/gdamore/tcell/v2 v2.8.1 // indirect
-	github.com/junegunn/fzf v0.58.0 // indirect
 	github.com/junegunn/go-shellwords v0.0.0-20240813092932-a62c48c52e97 // indirect
 	github.com/lucasb-eyer/go-colorful v1.2.0 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/mattn/go-runewidth v0.0.16 // indirect
+	github.com/ncruces/go-strftime v1.0.0 // indirect
+	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
 	github.com/rivo/uniseg v0.4.7 // indirect
-	golang.org/x/sys v0.29.0 // indirect
+	golang.org/x/sys v0.42.0 // indirect
 	golang.org/x/term v0.28.0 // indirect
 	golang.org/x/text v0.21.0 // indirect
+	modernc.org/libc v1.72.0 // indirect
+	modernc.org/mathutil v1.7.1 // indirect
+	modernc.org/memory v1.11.0 // indirect
 )