feat(builtin-models): add YAML-based declarative config with ${ENV} interpolation

[jackson-jia-914]

Description

Today built-in models can only be added by inserting rows into the models table via SQL (see docs/BUILTIN_MODELS.md). For operators who manage WeKnora through docker-compose and .env, this means hand-crafting SQL with secrets inlined and re-running it on every deployment.

This PR adds a declarative alternative built around a deliberate split between two files:

• config/builtin_models.yaml (committable) declares the shape of the built-in models — id, type, provider, and which env vars feed which fields.
• .env (gitignored) provides the values — API keys, base URLs, deployment-specific model names.

Any string field in YAML may reference an env var with ${NAME}. On startup the YAML is interpolated against os.Getenv and each entry is UPSERT-ed into the models table (is_builtin=true) by stable id. The split means operators can version-control their model topology without leaking secrets, and rotating an API key is a single .env edit + restart.

Implementation:

• internal/types/builtin_models_config.go (new) — loader, ${ENV} interpolation (os.Getenv only; no shell-style defaults), and per-entry GORM clause.OnConflict { Columns: id, UpdateAll: true } upsert. created_at is preserved on conflict; every other field refreshes. Unset env vars are intentionally left as the literal ${NAME} placeholder so misconfiguration surfaces visibly in provider calls instead of failing silently with an empty token.
• internal/container/container.go — calls LoadBuiltinModelsConfig once, post-migration. The loader treats file-missing / is-a-directory / parse error / per-entry upsert error all as warnings; startup never aborts because of YAML.
• internal/types/model.go — BeforeCreate now generates a UUID only when ID == "", so YAML-declared stable ids are preserved as the UPSERT key. API-driven model creation (which never sets ID) is unaffected.
• internal/config/config.go — new ConfigDir() helper exposes the directory of the loaded config.yaml so the builtin-models loader resolves builtin_models.yaml next to it without re-implementing viper's search rules. Overridable with BUILTIN_MODELS_CONFIG=/abs/path.
• docker-compose.yml — adds env_file: [{ path: .env, required: false }] so variables referenced by ${ENV} in YAML reach the container automatically without listing each one under environment:. required: false keeps fresh clones working with no .env file. Also adds a commented-out volume mount line for config/builtin_models.yaml as an opt-in hint.
• config/builtin_models.yaml.example (new) — generic template with inline documentation, covering both ${ENV}-driven and literal-value styles. The non-example path (config/builtin_models.yaml) is intentionally NOT shipped; operators copy the example and edit.
• .env.example — adds a commented "Built-in Models" section pointing at builtin_models.yaml.example and showing a reference set of LLM / Embedding / Rerank variable names. The block is comment-only so deployments that don't enable YAML are unaffected, and the wording calls out that the variable names are operator-defined (the YAML decides which names matter), not reserved keys.
• docs/BUILTIN_MODELS.md — rewritten with YAML as the recommended path, SQL retained as an alternative.

The feature is fully opt-in. Without config/builtin_models.yaml mounted, behavior is identical to before — the loader logs a single "skipping" line and returns.

Type of Change

• ✨ New feature

Testing

• Start without mounting config/builtin_models.yaml → log shows Built-in models config not present ... skipping.; existing SQL-seeded built-in models continue to work.
• Mount a YAML with three entries (LLM / Embedding / Rerank) referencing ${ENV} vars set in .env → on startup the three rows appear in models with is_builtin=true and the resolved values; restart is idempotent (same rows, no duplicates).
• Set an entry's env var to an unset name → the literal ${VAR_NAME} is persisted, and the resulting 401/connection error from the provider points clearly at the unset variable.
• Feed deliberately malformed YAML → startup logs a parse-error warning and continues without aborting.

Checklist

• Self-reviewed the code
• Added/updated tests covering the change
• Updated related documentation (docs/BUILTIN_MODELS.md, new config/builtin_models.yaml.example, .env.example)
• Breaking changes are clearly called out in the description above

Screenshots / Recordings

N/A — backend / config-only change.