Add public feed, admin area, self-registration, visibility & hashtags

- Public feed (/) with infinite scroll via Intersection Observer
- Self-registration (/register)
- Admin area (/admin/entries, /admin/users) with user management
- journal_entries: visibility (public/private) + hashtags fields
- users: is_admin flag
- DB schema updated (recreate DB to apply)
- CI: run go test via docker run (golang:1.25-alpine) — fixes 'go not found'

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/api/static/style.css

@@ -52,3 +52,17 @@ h2 { font-size: 1rem;   font-weight: normal; letter-spacing: .05em; }
 
 /* Login */
 .login-box { max-width: 360px; margin: 4rem auto; }
+
+/* Error message */
+.error { color: #c44; }
+
+/* Hashtags */
+.hashtags { margin-top: .3rem; }
+.tag { font-size: .75rem; background: var(--pico-muted-background-color); padding: .1rem .4rem; border-radius: 999px; margin-right: .2rem; }
+
+/* Visibility badge */
+.badge-public { font-size: .7rem; background: #264; color: #8f8; padding: .1rem .4rem; border-radius: 4px; vertical-align: middle; }
+
+/* Delete button */
+.btn-delete { background: none; border: 1px solid #c44; color: #c44; padding: .2rem .6rem; cursor: pointer; font-size: .8rem; border-radius: 4px; }
+.btn-delete:hover { background: #c44; color: #fff; }