feat(otel): OTel Collector receiver plugin for Flagon JSON

## Summary

Build a custom OTel Collector receiver (`flagonreceiver`) that accepts Flagon's existing JSON format and converts it to OTLP LogRecords server-side.

## Motivation

For users who can't change the client (e.g., already deployed v2.x UserALE) but want OTel on the backend.

## Design

- Written in Go (Collector plugin requirement)
- Accepts POST JSON on configurable HTTP path
- Maps Flagon JSON fields → OTLP LogRecords
- Registers as a standard Collector component
- Could be contributed to `opentelemetry-collector-contrib`

## Structure

```
collector/flagonreceiver/
factory.go
receiver.go
config.go
README.md
```

## Priority

Lower priority than client-side OTLP exporter (#140). Build when there's user demand.

## Depends On

- #140 (OTLP exporter — proves the attribute mapping works)

Start by reading issue #140 to understand the attribute mapping and dependency, then review the proposed collector/flagonreceiver/factory.go, receiver.go, config.go, and README.md structure. Define the receiver's HTTP path, Flagon JSON to OTLP LogRecord behavior, standard component registration, and documentation before implementation; done means the receiver accepts the existing JSON format and produces the expected OTLP records.

Written by the indexing model from the issue text.