Initial lookbook implementation

Pinterest-style visual bookmarking app with:
- URL metadata extraction (OG/Twitter meta, oEmbed fallback)
- Image caching in Postgres with 480px thumbnails
- Multi-tag filtering with Ctrl/Cmd for OR mode
- Fuzzy tag suggestions and inline tag editing
- Browser console auth() with first-use password setup
- Brutalist UI with Commit Mono font and Pico CSS
- Light/dark mode via browser preference

vendor/github.com/sethvargo/go-retry/backoff_exponential.go

@@ -0,0 +1,47 @@
+package retry
+
+import (
+	"context"
+	"math"
+	"sync/atomic"
+	"time"
+)
+
+type exponentialBackoff struct {
+	base    time.Duration
+	attempt uint64
+}
+
+// Exponential is a wrapper around Retry that uses an exponential backoff. See
+// NewExponential.
+func Exponential(ctx context.Context, base time.Duration, f RetryFunc) error {
+	return Do(ctx, NewExponential(base), f)
+}
+
+// NewExponential creates a new exponential backoff using the starting value of
+// base and doubling on each failure (1, 2, 4, 8, 16, 32, 64...), up to max.
+//
+// Once it overflows, the function constantly returns the maximum time.Duration
+// for a 64-bit integer.
+//
+// It panics if the given base is less than zero.
+func NewExponential(base time.Duration) Backoff {
+	if base <= 0 {
+		panic("base must be greater than 0")
+	}
+
+	return &exponentialBackoff{
+		base: base,
+	}
+}
+
+// Next implements Backoff. It is safe for concurrent use.
+func (b *exponentialBackoff) Next() (time.Duration, bool) {
+	next := b.base << (atomic.AddUint64(&b.attempt, 1) - 1)
+	if next <= 0 {
+		atomic.AddUint64(&b.attempt, ^uint64(0))
+		next = math.MaxInt64
+	}
+
+	return next, false
+}