## Summary
Move server-managed environments from sibling TOML files into SQLite,
matching the storage model already used by variables and secrets.
This adds:
- an `environments` SQLite table with DB-level validation for IDs,
revisions, providers, network modes, booleans, and JSON fields
- a SQLite-backed `EnvironmentStore` with cached synchronous reads,
transactional create/replace/delete, synthetic unpersisted `local`, and
`default` as an ordinary seeded row users can delete
- one-time legacy import from `environments/*.toml` next to the active
server `settings.toml`, including relative Dockerfile path inlining and
backup rename to `environments.imported-<timestamp>.bak`
- install/test/CLI seeding of `default` directly into SQLite instead of
writing `environments/default.toml`
- docs updates for API/SQLite-managed server environments and legacy
import behavior
The REST API shape is unchanged; path Dockerfile sources remain rejected
over the environments API.
## Testing
- `cargo nextest run -p fabro-db -p fabro-environment` - 15 passed
- `cargo nextest run -p fabro-server --features test-support
environments` - 16 passed
- `cargo nextest run -p fabro-server --features test-support install` -
60 passed
- `cargo nextest run -p fabro-server --features test-support
create_run_rejects_disabled_sandbox_provider` - 1 passed
- `cargo nextest run -p fabro-server --features test-support
system_sandbox_provider` - 2 passed
- `cargo nextest run -p fabro-cli install` - 132 passed
- `cargo +nightly-2026-04-14 fmt --check --all`
- `cargo +nightly-2026-04-14 clippy --workspace --all-targets -- -D
warnings`
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