Harden storage&cleanup, set up portable dist.

[akenshaw]

This PR started as an extension of the data cleanup subsystem but grew into a broader set of improvements across storage, config, documentation, and release automation.

Highlights

Cleanup & storage hardening

• Adds max_storage_mb — a hard cap on DuckDB file size (default 4 GiB).
• When exceeded, oldest trades are purged even if within the time-based retention window.
• Validates retention hours at startup, retries VACUUM on failure, and fixes an off-by-one in the purge delay.
• Separates initial cleanup pass from the periodic scheduler so startup is faster.
• Introduces headroom fraction to prevent hitting the cap during a flush.

Domain-typed config

• Replaces raw String/u64 values with newtypes:
  • BearerToken
  • RetentionHours
  • StorageBytes
• Makes the config self-documenting and harder to misuse.

Portable binary & distribution

• Config file now defaults to next to the binary (not CWD), so the whole app lives in one folder.
• Pass --config to override.
• data_dir relative paths are resolved from the config file's parent directory — everything stays self-contained.
• First-run template generation writes config.toml next to the binary; re-run to start.
• Release automation with cargo-dist release.yml:
  • Builds for Linux (x86 + ARM), macOS (Intel + Silicon), and Windows.
  • Archives include the binary, README.md, LICENSE, and config.example.toml.
  • Triggered by pushing a tag like v0.1.0.

Documentation overhaul

• Added full documented sections for:
  • GET /status
  • GET /pairs
  • GET /trades
  • GET /trades.arrow
  • GET /exchanges
• Includes real response examples.
• Cleaned up config template comments — relative paths now note they resolve from config directory.