fix(ingest): stop rolling the EC2 fleet on every merge and right-size hosts - #1716

🟢 Confidence 10/10 · safe to merge
Declarative infra change; the stamped revision moves with the apps/ingest image context, and env.test.ts pins the new lockstep list.
_{no findings · tests covered · risk medium}

Stops the ingest EC2 fleet from rolling on merges that leave apps/ingest alone by stamping that commit instead of the deployed one, drops ingest from the revision-lockstep list, and halves the prd ingest hosts (c7gd.medium, previews on a t4g.small). Safe to merge.

• INGEST_COMMIT_SHA (last commit touching apps/ingest) becomes the gateway's COMMIT_SHA
• Ingest leaves PRD_LOCKSTEP_REVISION_SERVICES
• US prd ingest moves to c7gd.medium with 1024/1536 tasks
• WAL_STORAGE gives a t4g.small a 4 GiB WAL on the root volume

Before merge

• Env var · Set INGEST_COMMIT_SHA in every environment, or confirm its default · apps/ingest/alchemy.run.ts:350
• Infra · Check every environment has what this config now expects · apps/ingest/alchemy.run.ts
• Manual · Confirm the 2a6e9529-5f73-4478-9fa0-432904ff15c8 "Prod revision skew" rule's SQL no longer expects ingest before that rule is re-enabled

_{7dae4b9 · Updated on every push. Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}