feat: add init sidecar for migrations and setup on compose up

Introduces a one-shot `init` service in docker-compose that runs Postgres
migrations, Neo4j index setup, and library-type seeding on every `up`.
Long-running services (`app`, `mcp`, `worker`) now depend on its
successful completion via `service_completed_successfully`, blocking the
stack on configuration errors (missing embedding model, dimension
mismatch, unreachable DB) rather than serving silent zero-result
searches.

Also standardizes reranker test fixtures to use the `/v1` OpenAI-style
base URL convention used across other service clients.

docs/PHASE_3_SEARCH_AND_RERANKING.md

@@ -61,6 +61,22 @@ POST http://pan.helu.ca:8400/v1/rerank
 }
 ```
 
+> **`LLMApi.base_url` convention.** Every Mnemosyne service client
+> (`EmbeddingClient`, `RerankerClient`, `vision.py`, `concepts.py`)
+> treats `base_url` as the **OpenAI-style `/v1` root** and appends a
+> path-only segment: `/embeddings`, `/rerank`, `/chat/completions`.
+> So a single `LLMApi` row with `base_url=http://pan.helu.ca:8400/v1`
+> serves both the embedding and the reranker endpoints — no per-purpose
+> duplication needed.
+>
+> Get this wrong (e.g. set `base_url=http://pan.helu.ca:8400` with no
+> `/v1`, or have a client prepend `/v1` locally) and you get a
+> double-prefixed URL like `…/v1/v1/rerank` that 404s silently —
+> `SearchService._rerank` catches the exception, the UI shows
+> "Re-rank: Skipped", and the search falls back to raw RRF order.
+> Check `results.reranker_skip_reason` on the search page for the
+> specific error.
+
 ## Deliverables
 
 ### 1. Search Service (`library/services/search.py`)