feat(auth): migrate user store from SQLite to PostgreSQL

Users were wiped on container recreation because they shared the
backend_data volume with ephemeral pharmacy/medicine data. Postgres
gets its own named volume (postgres_data) that survives deploys.

- Add postgres:16-alpine service + postgres_data volume to compose
- Add pg + connect-pg-simple deps
- userDbGet/userDbRun helpers: PG when PG_URL set, SQLite fallback
- Sessions use connect-pg-simple when pgPool available
- create-admin.js: PG-aware, SQLite fallback for local dev
- push_subscriptions.user_id: plain INTEGER (cross-DB FK dropped)
- Tests unchanged — no PG_URL in test env = SQLite fallback

Set PG_PASSWORD in server .env before next deploy.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/package-lock.json

@@ -11,11 +11,13 @@
       "dependencies": {
         "axios": "^1.6.0",
         "bcrypt": "^5.1.1",
+        "connect-pg-simple": "^10.0.0",
         "connect-sqlite3": "^0.9.16",
         "cors": "^2.8.5",
         "express": "^4.18.2",
         "express-rate-limit": "^8.5.2",
         "express-session": "^1.17.3",
+        "pg": "^8.13.0",
         "redis": "^4.6.0",
         "sqlite3": "^5.1.6",
         "web-push": "^3.6.7"
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       "license": "MIT"
     },
+    "node_modules/connect-pg-simple": {
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+      "resolved": "https://registry.npmjs.org/connect-pg-simple/-/connect-pg-simple-10.0.0.tgz",
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+      "dependencies": {
+        "pg": "^8.12.0"
+      },
+      "engines": {
+        "node": "^18.18.0 || ^20.9.0 || >=22.0.0"
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       "resolved": "https://registry.npmjs.org/connect-sqlite3/-/connect-sqlite3-0.9.16.tgz",
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+        "pg-pool": "^3.14.0",
+        "pg-protocol": "^1.14.0",
+        "pg-types": "2.2.0",
+        "pgpass": "1.0.5"
+      },
+      "engines": {
+        "node": ">= 16.0.0"
+      },
+      "optionalDependencies": {
+        "pg-cloudflare": "^1.4.0"
+      },
+      "peerDependencies": {
+        "pg-native": ">=3.0.1"
+      },
+      "peerDependenciesMeta": {
+        "pg-native": {
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+        }
+      }
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+    },
+    "node_modules/pg-int8": {
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+      "resolved": "https://registry.npmjs.org/pg-int8/-/pg-int8-1.0.1.tgz",
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+      "peerDependencies": {
+        "pg": ">=8.0"
+      }
+    },
+    "node_modules/pg-protocol": {
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+      "resolved": "https://registry.npmjs.org/pg-protocol/-/pg-protocol-1.14.0.tgz",
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+    },
+    "node_modules/pg-types": {
+      "version": "2.2.0",
+      "resolved": "https://registry.npmjs.org/pg-types/-/pg-types-2.2.0.tgz",
+      "integrity": "sha512-qTAAlrEsl8s4OiEQY69wDvcMIdQN6wdz5ojQiOy6YRMuynxenON0O5oCpJI6lshc6scgAY8qvJ2On/p+CXY0GA==",
+      "dependencies": {
+        "pg-int8": "1.0.1",
+        "postgres-array": "~2.0.0",
+        "postgres-bytea": "~1.0.0",
+        "postgres-date": "~1.0.4",
+        "postgres-interval": "^1.1.0"
+      },
+      "engines": {
+        "node": ">=4"
+      }
+    },
+    "node_modules/pgpass": {
+      "version": "1.0.5",
+      "resolved": "https://registry.npmjs.org/pgpass/-/pgpass-1.0.5.tgz",
+      "integrity": "sha512-FdW9r/jQZhSeohs1Z3sI1yxFQNFvMcnmfuj4WBMUTxOrAyLMaTcE1aAMBiTlbMNaXvBCQuVi0R7hd8udDSP7ug==",
+      "dependencies": {
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+      }
+    },
     "node_modules/picocolors": {
       "version": "1.1.1",
       "resolved": "https://registry.npmjs.org/picocolors/-/picocolors-1.1.1.tgz",
@@ -5041,6 +5135,41 @@
         "node": ">=8"
       }
     },
+    "node_modules/postgres-array": {
+      "version": "2.0.0",
+      "resolved": "https://registry.npmjs.org/postgres-array/-/postgres-array-2.0.0.tgz",
+      "integrity": "sha512-VpZrUqU5A69eQyW2c5CA1jtLecCsN2U/bD6VilrFDWq5+5UIEVO7nazS3TEcHf1zuPYO/sqGvUvW62g86RXZuA==",
+      "engines": {
+        "node": ">=4"
+      }
+    },
+    "node_modules/postgres-bytea": {
+      "version": "1.0.1",
+      "resolved": "https://registry.npmjs.org/postgres-bytea/-/postgres-bytea-1.0.1.tgz",
+      "integrity": "sha512-5+5HqXnsZPE65IJZSMkZtURARZelel2oXUEO8rH83VS/hxH5vv1uHquPg5wZs8yMAfdv971IU+kcPUczi7NVBQ==",
+      "engines": {
+        "node": ">=0.10.0"
+      }
+    },
+    "node_modules/postgres-date": {
+      "version": "1.0.7",
+      "resolved": "https://registry.npmjs.org/postgres-date/-/postgres-date-1.0.7.tgz",
+      "integrity": "sha512-suDmjLVQg78nMK2UZ454hAG+OAW+HQPZ6n++TNDUX+L0+uUlLywnoxJKDou51Zm+zTCjrCl0Nq6J9C5hP9vK/Q==",
+      "engines": {
+        "node": ">=0.10.0"
+      }
+    },
+    "node_modules/postgres-interval": {
+      "version": "1.2.0",
+      "resolved": "https://registry.npmjs.org/postgres-interval/-/postgres-interval-1.2.0.tgz",
+      "integrity": "sha512-9ZhXKM/rw350N1ovuWHbGxnGh/SNJ4cnxHiM0rxE4VN41wsg8P8zWn9hv/buK00RP4WvlOyr/RBDiptyxVbkZQ==",
+      "dependencies": {
+        "xtend": "^4.0.0"
+      },
+      "engines": {
+        "node": ">=0.10.0"
+      }
+    },
     "node_modules/prebuild-install": {
       "version": "7.1.3",
       "resolved": "https://registry.npmjs.org/prebuild-install/-/prebuild-install-7.1.3.tgz",
@@ -5712,6 +5841,14 @@
         "source-map": "^0.6.0"
       }
     },
+    "node_modules/split2": {
+      "version": "4.2.0",
+      "resolved": "https://registry.npmjs.org/split2/-/split2-4.2.0.tgz",
+      "integrity": "sha512-UcjcJOWknrNkF6PLX83qcHM6KHgVKNkV62Y8a5uYDVv9ydGQVwAHMKqHdJje1VTWpljG0WYpCDhrCdAOYH4TWg==",
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+      }
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     "node_modules/sprintf-js": {
       "version": "1.0.3",
       "resolved": "https://registry.npmjs.org/sprintf-js/-/sprintf-js-1.0.3.tgz",
@@ -6389,6 +6526,14 @@
         "node": "^12.13.0 || ^14.15.0 || >=16.0.0"
       }
     },
+    "node_modules/xtend": {
+      "version": "4.0.2",
+      "resolved": "https://registry.npmjs.org/xtend/-/xtend-4.0.2.tgz",
+      "integrity": "sha512-LKYU1iAXJXUgAXn9URjiu+MWhyUXHsvfp7mcuYm9dSUKK0/CjtrUwFAxD82/mCWbtLsGjFIad0wIsod4zrTAEQ==",
+      "engines": {
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+      }
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     "node_modules/y18n": {
       "version": "5.0.8",
       "resolved": "https://registry.npmjs.org/y18n/-/y18n-5.0.8.tgz",