Enhance ledger operations and referral lifecycle

Add traceable ledger filtering and permanent referral codes so operators can investigate credit activity without weakening immutable accounting guarantees.

src/main/resources/db/migration/V19__admin_ledger_operations.sql

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+ALTER TABLE credit_ledger
+    MODIFY reference_id CHAR(36) CHARACTER SET ascii COLLATE ascii_bin NULL;
+
+ALTER TABLE provider_requests
+    MODIFY reservation_id CHAR(36) CHARACTER SET ascii COLLATE ascii_bin NULL;
+
+CREATE INDEX idx_credit_ledger_amount_id
+    ON credit_ledger (amount_delta, id);
+
+CREATE INDEX idx_credit_ledger_user_amount_id
+    ON credit_ledger (user_id, amount_delta, id);
+
+CREATE INDEX idx_provider_requests_capability_source_reservation
+    ON provider_requests (capability, request_source, reservation_id);
+
+CREATE INDEX idx_provider_requests_source_capability_reservation
+    ON provider_requests (request_source, capability, reservation_id);

src/main/resources/db/migration/V20__permanent_referral_codes.sql

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+-- Keep historical campaign-scoped codes as valid aliases while selecting exactly
+-- one permanent code for every account. New writes claim this mapping atomically.
+CREATE TABLE referral_owner_codes (
+    owner_user_id VARCHAR(36) CHARACTER SET ascii COLLATE ascii_bin NOT NULL,
+    code_id CHAR(36) NOT NULL,
+    created_at DATETIME(6) NOT NULL DEFAULT CURRENT_TIMESTAMP(6),
+    PRIMARY KEY (owner_user_id),
+    UNIQUE KEY uk_referral_owner_codes_code (code_id),
+    CONSTRAINT fk_referral_owner_codes_code
+        FOREIGN KEY (code_id) REFERENCES referral_codes (id) ON DELETE CASCADE
+) ENGINE = InnoDB;
+
+-- Preserve the code most recently distributed by the previous implementation.
+-- Older codes remain in referral_codes so already-shared links never break.
+INSERT INTO referral_owner_codes (owner_user_id, code_id, created_at)
+SELECT candidate.owner_user_id, candidate.id, candidate.created_at
+FROM referral_codes candidate
+WHERE NOT EXISTS (
+    SELECT 1
+    FROM referral_codes newer
+    WHERE newer.owner_user_id = candidate.owner_user_id
+      AND (
+          newer.created_at > candidate.created_at
+          OR (
+              newer.created_at = candidate.created_at
+              AND newer.id > candidate.id
+          )
+      )
+);