chore(release): 0.20.0 -- DCA exempt from the per-asset rail; dca plan and its read-only card

[eaitbrahim]

Version bump only: all 8 pyprojects, their == pins, and uv.lock go from 0.19.0 to 0.20.0. That's 9 files, the same shape as #845. uv lock --check passes, and there's no schema change since 0.19.0.

MINOR, because rail 6 changes scope for DCA (#869), and a new command and route land (#846, #850).

• feat(dca): keel dca plan -- propose a DCA schedule within the venue plan's cap #846: keel dca plan
• feat(web): read-only DCA plan card on /rules #850: GET /api/dca-plan and the read-only card on /rules
• fix(rails): exempt DCA buys from rail 6; dca plan checks rails 3 and 5 (#853) #869 (DCA concentration: exempt DCA buys from rail 6, and warn in keel dca plan on the rails that still bind #853): DCA buys are exempt from rail 6; the plan blocks on rail 3 and warns on rail 5
• feat(dca): a cap-blocked plan names the largest passing budget and buffer (#854) #867 (keel dca plan: a cap-blocked plan names the largest budget and buffer that would pass #854), fix(dca): the passing-value suggestion clears rails 3 and 2 as well as 14 (#874) #876 (keel dca plan: the rail-14 suggestion can name a value rail 2 or rail 3 still blocks #874): a blocked plan names the values that pass rails 14, 3 and 2
• feat(web): cache the DCA plan's admission screen for 5 minutes (#856) #870 (Cache the admission screen behind /api/dca-plan (5-minute TTL) #856): the admission screen is cached for 5 minutes
• chore(deploy): sync live-rules.json with the live rule set (#855) #858 (Sync deploy/live-rules.json with the live rule set #855): the live-rules manifest is synced
• docs(spec): DCA sleeve sell side -- trims, rebalancing, reverse DCA, protective exits, rotation (#857) #859: the sell-side design spec

Local: pytest 7074 passed, 3 skipped; ruff and mypy clean.

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