Honour EMBEDDER_PROVIDER, and allow model/dimensions from the environment

[a-optic]

A documented variable that does nothing

.env.example ships EMBEDDER_PROVIDER=ollama and tells the reader to "set EMBEDDER_PROVIDER and provide the matching API key below". EmbedderConfig only ever reads provider from config/settings.yaml, so setting the documented variable has no effect.

The harder half: model and dimensions

model and dimensions are config-file-only too, and in a container deployment config/settings.yaml is baked into the image at build time. Running a different embedding model therefore means editing a tracked file and rebuilding — and that file is then the one most likely to conflict on every upgrade, for anyone not using the default model.

The change

All three now follow the same env-over-config precedence max_chars and chunk_overlap already use:

EMBEDDER_PROVIDER    # previously documented but inert
EMBEDDER_MODEL       # new
EMBEDDER_DIMENSIONS  # new

Two details worth calling out:

• A set-but-empty variable is treated as unset rather than as a configured empty string. .env files routinely carry blank placeholders, and letting one through would silently select no provider at all.
• dimensions is validated as a positive integer, matching the existing max_chars / chunk_overlap checks.

.env.example documents the two new variables, including the constraint that they must agree with the width of the embedding column.

Verification

Against a running deployment:

no env set                       -> settings.yaml wins   (arctic-embed2, 1024)
EMBEDDER_MODEL / _DIMENSIONS set -> env wins             (mxbai-embed-large, 512)
EMBEDDER_MODEL=""   or "   "     -> falls back to settings.yaml
EMBEDDER_DIMENSIONS=0            -> ValueError

Scope

This does not change where the SQL dimension comes from — schema.sql and migration 015_embedding_dim_change.sql still hardcode 768, so a deployment on a different width still manages the column itself. Happy to follow up on driving that from the same setting if you'd want it.