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/db/schema.sql

@@ -5,6 +5,7 @@ CREATE TABLE IF NOT EXISTS users (
     user_id       TEXT PRIMARY KEY DEFAULT gen_random_uuid()::text,
     username      TEXT NOT NULL UNIQUE,
     password_hash TEXT NOT NULL,
+    is_admin      BOOLEAN NOT NULL DEFAULT false,
     created_at    TIMESTAMPTZ NOT NULL DEFAULT NOW()
 );
 
@@ -78,9 +79,12 @@ CREATE TABLE IF NOT EXISTS journal_entries (
     description  TEXT NOT NULL DEFAULT '',
     lat          DOUBLE PRECISION,
     lon          DOUBLE PRECISION,
+    visibility   TEXT NOT NULL DEFAULT 'private',
+    hashtags     TEXT[] NOT NULL DEFAULT '{}',
     created_at   TIMESTAMPTZ NOT NULL DEFAULT NOW()
 );
 CREATE INDEX IF NOT EXISTS journal_entries_user_date_idx ON journal_entries(user_id, entry_date);
+CREATE INDEX IF NOT EXISTS journal_entries_public_idx ON journal_entries(visibility, created_at DESC);
 
 CREATE TABLE IF NOT EXISTS journal_images (
     image_id      TEXT PRIMARY KEY DEFAULT gen_random_uuid()::text,