feat(storage): support BLOGWATCHER_DB override

[maciej]

Why

Blogwatcher currently stores everything in a single ~/.blogwatcher/blogwatcher.db.

That makes isolation between independent blog lists difficult and increases risk for broad operations like read-all (including agent-driven usage), where one context can accidentally affect another.

What changed

• Added BLOGWATCHER_DB as a default DB path override.
• DefaultDBPath() now:
  • uses BLOGWATCHER_DB when it is set to a non-empty value
  • falls back to ~/.blogwatcher/blogwatcher.db when unset/empty
• Kept CLI behavior unchanged (OpenDatabase("") still works across commands; it now resolves through the env-aware default path).
• Updated docs to describe the override and isolation use case.

Tests

• Added TestDefaultDBPathUsesEnvOverride
• Added TestDefaultDBPathFallsBackWhenEnvIsEmpty
• Verified go test ./...

Smoke test

Validated against:

• Releases page: https://github.com/Hyaxia/blogwatcher/releases
• Feed: https://github.com/Hyaxia/blogwatcher/releases.atom

Using isolated DB path:

• BLOGWATCHER_DB=/tmp/blogwatcher-smoke.db ... add
• ... scan
• ... articles --all

Observed 2 release entries (v0.0.2, v0.0.1) stored in the isolated DB, and a separate DB path showed no shared articles.

Backward compatibility

No breaking change: if BLOGWATCHER_DB is not set (or empty), behavior remains unchanged.