feat(api): API keys + Redis rate limiting + usage stats

Add an optional API-key layer to the public read-only API. Keys grant
higher per-minute rate limits and attribute usage; anonymous callers are
still allowed at a lower IP-based budget.

- migration 0008_api_key: api_key table (sha256 hash only, plaintext shown once)
- apikey pkg: key generation + hashing
- ratelimit pkg: Redis fixed-window limiter + per-key usage counters; fails open
- public API middleware: X-API-Key / Bearer auth, X-RateLimit-* headers, 429+Retry-After
- admin: issue/list/revoke keys + usage view (API + UI tab)

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

api/internal/apikey/apikey.go

@@ -0,0 +1,49 @@
+// Package apikey handles generation and hashing of public-API keys.
+//
+// A key looks like "og_live_<random>". Only the SHA-256 hash is ever persisted;
+// the plaintext is returned once at creation time and cannot be recovered.
+package apikey
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/hex"
+	"strings"
+)
+
+// Prefix is the human-readable scheme prefix every key carries.
+const Prefix = "og_live_"
+
+// prefixLen is how many leading characters (including Prefix) are stored in
+// api_key.key_prefix for identifying a key without revealing its secret.
+const prefixLen = 12
+
+// Generate returns a new random key (plaintext), its SHA-256 hash, and a short
+// display prefix. The plaintext must be shown to the caller exactly once.
+func Generate() (key, hash, displayPrefix string, err error) {
+	buf := make([]byte, 24)
+	if _, err = rand.Read(buf); err != nil {
+		return "", "", "", err
+	}
+	// URL-safe, no padding => stable, copy-pasteable token body.
+	body := base64.RawURLEncoding.EncodeToString(buf)
+	key = Prefix + body
+	hash = Hash(key)
+	displayPrefix = key
+	if len(displayPrefix) > prefixLen {
+		displayPrefix = displayPrefix[:prefixLen]
+	}
+	return key, hash, displayPrefix, nil
+}
+
+// Hash returns the hex-encoded SHA-256 of a key, used for storage and lookup.
+func Hash(key string) string {
+	sum := sha256.Sum256([]byte(strings.TrimSpace(key)))
+	return hex.EncodeToString(sum[:])
+}
+
+// Looks like a key issued by this service (cheap pre-check before hashing).
+func IsWellFormed(key string) bool {
+	return strings.HasPrefix(key, Prefix) && len(key) > len(Prefix)+8
+}

api/internal/apikey/apikey_test.go

@@ -0,0 +1,60 @@
+package apikey
+
+import "testing"
+
+func TestGenerate(t *testing.T) {
+	key, hash, prefix, err := Generate()
+	if err != nil {
+		t.Fatalf("Generate: %v", err)
+	}
+	if !IsWellFormed(key) {
+		t.Fatalf("generated key not well-formed: %q", key)
+	}
+	if Hash(key) != hash {
+		t.Fatalf("Hash(key) != returned hash")
+	}
+	if len(prefix) != prefixLen || key[:prefixLen] != prefix {
+		t.Fatalf("prefix %q not a %d-char prefix of key %q", prefix, prefixLen, key)
+	}
+	if len(hash) != 64 {
+		t.Fatalf("hash not hex sha-256: %q", hash)
+	}
+}
+
+func TestGenerateUnique(t *testing.T) {
+	seen := map[string]bool{}
+	for i := 0; i < 100; i++ {
+		k, _, _, err := Generate()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if seen[k] {
+			t.Fatalf("duplicate key generated: %q", k)
+		}
+		seen[k] = true
+	}
+}
+
+func TestHashStableAndTrimmed(t *testing.T) {
+	if Hash("og_live_abc") != Hash("  og_live_abc  ") {
+		t.Fatal("Hash should ignore surrounding whitespace")
+	}
+	if Hash("a") == Hash("b") {
+		t.Fatal("distinct inputs must hash differently")
+	}
+}
+
+func TestIsWellFormed(t *testing.T) {
+	cases := map[string]bool{
+		"og_live_abcdefghijkl": true,  // body longer than 8 chars
+		"og_live_":             false, // empty body
+		"og_live_abc":          false, // body too short
+		"nope_abcdefghijkl":    false, // wrong prefix
+		"":                     false,
+	}
+	for in, want := range cases {
+		if got := IsWellFormed(in); got != want {
+			t.Errorf("IsWellFormed(%q) = %v, want %v", in, got, want)
+		}
+	}
+}