Add README and proper favicon/PWA icon set

README covers what alembic does, prerequisites, a full deploy quickstart,
credential setup, and a page-by-page usage tour, written for someone
running Docker but not necessarily comfortable coding or sysadmin work.

Favicon/icons were prepared in a separate theme scratch dir but never
wired in; moved them into app/static and pointed base.html at the full
icon set (SVG, PNG fallbacks, apple-touch-icon, manifest) instead of the
single inline data-URI SVG it had before.

app/static/site.webmanifest

@@ -0,0 +1,13 @@
+{
+  "name": "Alembic",
+  "short_name": "Alembic",
+  "description": "Playlist ingestion, tagging, dedupe, and library tools for Alembic.",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#08090d",
+  "theme_color": "#08090d",
+  "icons": [
+    { "src": "/static/icon-192.png", "sizes": "192x192", "type": "image/png" },
+    { "src": "/static/icon-512.png", "sizes": "512x512", "type": "image/png" }
+  ]
+}