docs(bootstrap): clarify three-step Docker first-boot flow

Rework README and docker-compose comments to document the deliberate
chicken-and-egg escape: the `init` sidecar now only runs `migrate` and
`load_library_types`, leaving `setup_neo4j_indexes` as a manual step
after the system embedding model is configured in `/admin/`. This
avoids making `app` unreachable on first boot when no embedding model
row exists yet, while preserving loud failure on dimension mismatch.

docs/mnemosyne.html

@@ -295,7 +295,7 @@ graph LR
 
             <div class="alert alert-warning border-start border-4 border-warning">
                 <h4><i class="bi bi-lightning"></i> Neo4j Indexes (managed by <code>setup_neo4j_indexes</code>)</h4>
-                <p>Created by the <code>init</code> sidecar on every <code>docker compose up</code>. Vector dimensions come from the system embedding model's <code>vector_dimensions</code> field — the command fails if no model is configured. Current production model: <strong>Pan Synesis · qwen3-vl-embedding-2b · 2048d</strong>.</p>
+                <p>Run manually after the first <code>docker compose up</code>, once the system embedding model has been configured in <code>/admin/llm_manager/llmmodel/</code>: <code>docker compose exec app python manage.py setup_neo4j_indexes</code>. Vector dimensions come from the model's <code>vector_dimensions</code> field — the command hard-fails if no such row exists, which is why it is <em>not</em> bundled into the <code>init</code> sidecar (doing so would make the admin unreachable on first boot). Current production model: <strong>Pan Synesis · qwen3-vl-embedding-2b · 2048d</strong>.</p>
                 <pre class="bg-light p-3 rounded mb-0"><code>// Chunk text+image embeddings (dimensions read from system embedding model)
 CREATE VECTOR INDEX chunk_embedding_index FOR (c:Chunk)
 ON (c.embedding) OPTIONS {indexConfig: {