Fixes#73
## Why
`Classifier.Classify` runs on every proxied request and recompiled the Blocklist/Patterns/Immutable/Mutable regex lists each time. Regex compilation is expensive and fully redundant.
## Changes
- Memoise compilation in a `sync.Map` keyed by pattern text (`compileCached`); each distinct pattern compiles once and is reused. Patterns that fail to compile are cached as a typed nil so they are not retried. No invalidation needed since the pattern text is the key.
## Validation
- `go test ./internal/proxy/` and `make e2e` pass.
Reviewed-on: #88
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>
unkinben
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