Config improvements: error handling and documentation updates

[gustavovalverde]

Summary

During the config-rs migration (PR #469), several pre-existing issues were identified that are out of scope for that PR but should be addressed.

Issues Found

1. main.rs unwraps config loading (potential panic)

Location: zainod/src/main.rs:35

match start_indexer(load_config(&config_path).unwrap()).await {

Config failures should be handled gracefully with proper error messages instead of panicking.

Suggested fix: Replace .unwrap() with proper error handling that logs the config error and exits gracefully.

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2. ZainodConfig::default() unwraps HOME-derived path (potential panic)

Location: zainod/src/config.rs:183

zebra_db_path: default_zebra_db_path().unwrap(),

If HOME environment variable is not set, this will panic when creating default config.

Suggested fix: Either:

• Use a fallback path (like /tmp/zebra/.cache)
• Make zebra_db_path an Option<PathBuf> with runtime validation
• Align with default_zaino_db_path() which already has a /tmp fallback

---

3. Documentation mentions no_db option that doesn't exist

Location: docs/use_cases.md:19

Unless the no_db option is set to true in the config file...

The code actually uses DatabaseSize::Gb(0) to indicate no database (zaino-state/src/local_cache/non_finalised_state.rs:375-378):

let no_db = match self.config.storage.database.size {
    zaino_common::DatabaseSize::Gb(0) => true,
    zaino_common::DatabaseSize::Gb(_) => false,
};

Suggested fix: Update documentation to reflect actual configuration method, e.g.:

Set storage.database.size = { gb = 0 } to disable local database caching...

[nachog00]

#854 fixes problems 1 and 2. I'm opening another issue to track problem 3