Add schema, handler, and proxy infrastructure for channel request details (#4541)

[stuartc]

Description

This PR adds the backend infrastructure needed for the channel request detail page. It extends the data model and proxy pipeline to capture richer request/response metadata that was previously discarded.

Schema changes:

• New columns on channel_events: request_query_string, request_body_size, response_body_size, request_send_us, response_duration_us
• Migrate request_headers and response_headers from text to native jsonb (with USING clause for in-place data conversion, handling NULL and malformed JSON)
• New columns on channel_requests: client_webhook_auth_method_id (FK with nilify_all) and client_auth_type (denormalized snapshot that survives auth method deletion)

Handler + proxy plug changes:

• Adapt handler to Philter 0.3.0 timing structure (result.timing.total_us replaces result.duration_us)
• Persist per-direction durations (send_us, recv_us) and body sizes from Philter observations
• Pass collect_timing: true to Philter for per-phase timing breakdown
• Pass conn.query_string through handler state to persist on events
• Identify which client auth method matched (not just pass/fail) and persist the method ID and type on the channel request
• Stop JSON-encoding headers — store as native jsonb arrays

Closes #4541

Validation steps

1. mix test test/lightning/channels/ — 75 tests, 0 failures
2. mix test test/lightning_web/plugs/channel_proxy_plug_test.exs — 49 tests, 0 failures
3. mix verify — 4713 tests, 0 failures

Additional notes for the reviewer

1. The headers jsonb migration (20260409100002) uses a CASE/USING clause to convert existing text data in-place. NULL values stay NULL; malformed JSON falls back to NULL.
2. empty_values: [] was added to the ChannelEvent changeset because request_query_string needs to preserve empty strings ("" means "no query params"). Since all ChannelEvent fields are machine-written by the handler, not user input, this is safe.
3. authenticate_client was refactored from boolean pass/fail to identifying the specific matched method, reusing Auth.valid_key?/Auth.valid_user? per-method to preserve constant-time comparison.

TODO

• Migration: new columns on channel_events (body sizes, durations, query string)
• Migration: convert request_headers and response_headers from text to jsonb
• Migration: add client_webhook_auth_method_id and client_auth_type to channel_requests
• Update ChannelEvent schema and changeset for new fields + jsonb headers
• Update ChannelRequest schema with client auth method association
• Adapt handler to Philter 0.3.0 timing structure
• Handler persists body sizes, per-direction durations, and query string
• Handler stores headers as native jsonb (no more Jason.encode!)
• Proxy plug passes collect_timing: true to Philter
• Proxy plug passes conn.query_string into handler state
• Proxy plug identifies matched client auth method and passes ID + type into handler state
• Test contracts for all schema, handler, and proxy plug changes
• All existing tests pass (4713 tests, 0 failures)
• Detail LiveView page (ChannelRequestLive.Show) with summary card, request/response sections, timing visualization, and context section
• Error message humanization module (transport + credential error codes → human-readable messages)
• Navigation wiring: link from channel logs table, breadcrumbs, route + feature gate
• Context function Channels.get_channel_request_for_project/2 with project-scoped query

AI Usage

• I have used Claude Code
• I have used another model
• I have not used AI

Pre-submission checklist

• I have performed an AI review of my code (we recommend using /review with Claude Code)
• I have implemented and tested all related authorization policies.
• I have updated the changelog.
• I have ticked a box in "AI usage" in this PR