[WIP] Implement storage schema for import sessions and transactions

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>feat: storage schema — import sessions, canonical transactions, merchant tables</issue_title>
<issue_description>## Summary
Design and implement the storage schema that underpins the entire application. Current storage is likely too shallow.

Schema

-- Import provenance
import_sessions(id, file_hash, filename, account_id, imported_at, total_rows, parsed_rows, error_rows)

-- Raw imported data (never modified)
raw_transactions(id, import_session_id, row_number, date, amount_cents, description, raw_data_json)

-- Resolved/canonical transactions
canonical_transactions(id, raw_transaction_id, date, amount_cents, merchant_id, category, confidence, reviewed)

-- Merchant resolution
merchants(id, canonical_name, category, created_at, updated_at)
merchant_aliases(id, merchant_id, alias_pattern, match_type, created_by)

-- Double-entry ledger
accounts(id, name, type, institution, currency, created_at)
postings(id, date, debit_account_id, credit_account_id, amount_cents, canonical_transaction_id, memo)

-- Recurring detection
recurring_series(id, merchant_id, account_id, interval_days, avg_amount_cents, confidence, status)

-- Review decisions
review_decisions(id, entity_type, entity_id, decision, decided_at, previous_value, new_value)

Principles

• Raw data is immutable — never modify imported rows
• All mutations go through canonical layer
• Every decision is logged in review_decisions
• Use PluresDB if available, SQLite as fallback

Acceptance

• Schema implemented with migrations
• Raw transactions preserved immutably
• All CRUD operations tested
• Review decisions create audit trail</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes feat: storage schema — import sessions, canonical transactions, merchant tables #66

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[kayodebristol]

Closing this PR to enforce the 1-PR-per-repo rule. Only one Copilot PR is allowed per repository at a time. The oldest PR (#48) will be kept active.