feat: Accept config values with !env_file in yaml

[contre95]

Summary

• Adds a new !env_file YAML tag to the config loader, alongside the existing !env_var tag. Where !env_var NAME reads the secret directly from env var NAME, !env_file NAME treats NAME's value as a file path and reads the secret from that file's (trimmed) contents.
• This follows the standard Docker/Kubernetes secrets convention (SOMETHING_FILE=/run/secrets/something), so secrets can be mounted as files instead of passed as plaintext env vars — useful for Docker Swarm/Compose secrets, Kubernetes Secret volumes, and avoiding secrets showing up in docker inspect, process env, or shell history.
• Applies to any secret field in the config, not just AcoustID — e.g. telegram.token, metadata.providers.discogs.secret, metadata.providers.acoustid.secret.
• Same failure semantics as !env_var: startup fails with a clear error if the env var is unset, the file can't be read, or the file is empty.

Changes

• src/features/config/manager.go — new !env_file branch in processEnvVarNodes.
• src/features/config/manager_test.go — new tests: !env_var + !env_file resolving correctly together, and the error path when the referenced file is missing.
• config.example.yaml, docs/features/config.md, README.md — document the new tag with examples for AcoustID/Discogs/Telegram.

Test plan

• CGO_ENABLED=0 go build ./...
• CGO_ENABLED=0 go test ./src/features/config/... (both new tests pass)
• Manually verified a config with secret: !env_file ACOUSTID_CLIENT_KEY_FILE resolves against a real file and fails cleanly when the file is absent/empty

Summary by CodeRabbit

• New Features

  • Added support for loading secret values from files using the !env_file configuration tag.
  • File paths are provided through environment variables, with trimmed file contents used as configuration values.
  • Startup now reports errors when referenced files are missing, unreadable, or empty.

• Documentation

  • Updated configuration documentation and examples with !env_file usage and Docker/Kubernetes secret conventions.

[coderabbitai]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 30d4006 and 27e0eeb.

📒 Files selected for processing (5)

• README.md
• config.example.yaml
• docs/features/config.md
• src/features/config/manager.go
• src/features/config/manager_test.go

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📝 Walkthrough

Walkthrough

The configuration manager now supports !env_file tags. It reads a file path from an environment variable and uses the trimmed file contents as the value. Tests and documentation cover successful resolution and startup errors.

Changes

Environment file configuration support

Layer / File(s)	Summary
Resolve environment file tags src/features/config/manager.go, src/features/config/manager_test.go	processEnvVarNodes resolves !env_file tags, validates environment variables and file contents, and reports errors. Tests cover successful resolution and missing files.
Document environment file configuration README.md, config.example.yaml, docs/features/config.md	Documentation describes !env_file, secret file paths, startup errors, and Docker/Kubernetes secret usage.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~15 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant ConfigManager
  participant Environment
  participant SecretFile
  ConfigManager->>Environment: Look up the !env_file variable
  Environment-->>ConfigManager: Return the file path
  ConfigManager->>SecretFile: Read the file
  SecretFile-->>ConfigManager: Return trimmed contents or an error

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Merge Risk: ⚪ Minimal · up to 27e0e

The new secret-file configuration behavior, error handling, tests, and documentation are aligned in the supplied change context. No merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 2 files. (3 skipped: 3 …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: adding support for !env_file configuration values in YAML.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 2 files. (3 skipped: 3 unsupported.)

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[contre95]

Attempts to address #183