refactor: remove analytics channel, use outlet batch dispatch directly

[fergusfinn]

Summary

• Removes the intermediate channel + accumulation loop between outlet and analytics writes
• Renames AnalyticsBatcher → AnalyticsWriter (no longer batches, just enriches + writes)
• Overrides handle_response_batch on AnalyticsHandler for direct batch processing
• Removes the analytics-batcher background task from BackgroundServices
• Increases outlet channel_capacity from 4096 → 16384 since outlet is now the sole buffer for billing data

Before

outlet → handle_response_batch (default, calls per-item)
  → AnalyticsHandler.handle_response × N
    → channel.send(RawAnalyticsRecord) × N
      → AnalyticsBatcher background task
        → re-accumulate from channel
        → enrich batch
        → transactional write

After

outlet → handle_response_batch (override)
  → AnalyticsHandler.handle_response_batch
    → extract RawAnalyticsRecords from slice
    → AnalyticsWriter.flush
      → enrich batch
      → transactional write

Net result: -161 lines, one fewer background task, one fewer channel, same enrichment/write/retry logic.

Test plan

• just lint rust passes
• just test rust passes (1053 tests, 0 failures)
• CI passes

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[Copilot]

Pull request overview

This PR refactors the Rust backend analytics/billing pipeline to remove the intermediate analytics channel + background batcher task, and instead process outlet’s response batches directly via an AnalyticsWriter that enriches and writes records transactionally.

Changes:

• Remove the analytics channel and background “analytics-batcher” task; rely on outlet batch dispatch.
• Rename AnalyticsBatcher → AnalyticsWriter and expose a flush(&[RawAnalyticsRecord]) API for batched writes.
• Update router/background service wiring to enable analytics without passing around an analytics sender; increase outlet buffer capacity.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
dwctl/src/test/mod.rs	Updates build_router call sites to match the new signature.
dwctl/src/request_logging/mod.rs	Stops re-exporting the removed AnalyticsBatcher.
dwctl/src/request_logging/batcher.rs	Refactors the former batcher into AnalyticsWriter and updates integration tests to call flush.
dwctl/src/request_logging/analytics_handler.rs	Implements direct handle_response_batch processing and delegates to AnalyticsWriter.
dwctl/src/lib.rs	Wires analytics handler directly in build_router, removes batcher background task setup, and increases outlet channel_capacity.

Comments suppressed due to low confidence (2)

dwctl/src/request_logging/batcher.rs:55

• The doc comment for RawAnalyticsRecord still says the record is "sent through the channel" and that enrichment happens in the "batcher", but this PR removes the channel and renames the component to AnalyticsWriter. Updating this comment will prevent confusion when reading the new architecture.

/// Raw analytics record sent through the channel (unenriched).
///
/// This contains only data that can be extracted from the request/response
/// without any database lookups. Enrichment happens in the batcher.
#[derive(Debug, Clone)]

dwctl/src/request_logging/mod.rs:10

• request_logging no longer re-exports the renamed writer type. Previously AnalyticsBatcher was exported from this module; after the rename, consider re-exporting AnalyticsWriter (or documenting the intended access path) to avoid an accidental public API breaking change.

pub use analytics_handler::AnalyticsHandler;
pub use models::{AiRequest, AiResponse, ParsedAIRequest};

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