feat(config): add body_size_limit to TOML config

[pgagnidze]

Summary

• Adds body_size_limit at both global and per-target level to TOML config, using the *int64 tri-state pattern already established for *bool fields in fix(config): resolve boolean inheritance bug for target overrides #40.
• Semantics match the net package and blackbox_exporter: nil = inherit from global, 0 = unlimited, >0 = cap.
• Default is net.DefaultBodySizeLimit (1 MiB) via viper, preserving the safe-by-default behavior introduced in fix(net): cap response body reads via configurable size limit #41.
• Propagates through the Lambda request boundary so remote probes honor the configured limit.
• Updates README and example-config.toml to document the new knob.

Follow-up to #41.

Test plan

• go build ./...
• go vet ./...
• go test ./...
• New TestBodySizeLimitInheritance covers inherit from global, explicit target override, and per-target 0 = unlimited
• New TestBodySizeLimitDefault covers the viper default (1 MiB) when neither global nor target sets the field
• Manual: verified all three semantics end-to-end with a 2 MiB test endpoint and a TOML config that exercised each case