Issue #3607898: Fix the remaining 1.0.0-alpha6 audit findings (mediums, lows, performance, docs, fr translations)

Follow-up to the alpha6 audit; the two Highs were #3607819 and #3607820. This collects the remaining findings: performance (per-language notify fan-out, Batch API for bulk cancel/migrate), the Mediums and Lows, the documentation refresh (Assignment to Audience, Recipient VO, dumb-channel mail README, new submodule READMEs), and the French translations. See the issue for the full list.